If he hadn't used his then girlfriend Tamsin's own e-mail address to make the purchase, he might well have been hacking freely to this day.
Tamsin, a librarian, is the other tragic figure in this story. For several years, McKinnon was living with her in a flat in her aunt's house in Hillfield Avenue, Hornsey, North London.
One can only imagine Tamsin's frustration (she has not responded to my requests for an interview) as his hacking obsession took over.
In late 2000, McKinnon even gave up his paid job as a computer administrator to devote himself to cyberspace infiltration. Soon, he stopped washing, became noctural, ate rarely, smoked marijuana and spent all day in a dressing-gown.
Understandably, Tamsin dropped him - although, out of fondness or insanity, she continued to share the flat with him. She really should have got out then, because his night-time activities were now alerting some very serious people.
Between early 2000 and autumn 2001, McKinnon hacked into 97 computer systems, belonging to the U.S. Army, Navy, Air Force, Department of Defense and NASA.
He could do this because their security was unbelievably lax. 'It was so easy,' he says today. 'I found out that the U.S. military use the Windows computer software system. And having realised this, I assumed it would probably be an easy hack if they hadn't secured it properly.'
McKinnon's cyber crowbar consisted of several readily available programmes 'glued' together by one of his own.
The package searched for 'blank passwords' - computers whose passwords hadn't been changed from their default setting. It was no harder than climbing a neighbour's fence. Online, he could scan '65,000 machines in little more than eight minutes'.
Once he had found a loophole, he was into the host network.
'It wasn't very clever,' he has said. 'There were no lines of defence. And I wasn't alone in doing it. Often, I could see that there were a number of other foreign hackers present at any one time.' So what did he see there?
One example he gives was an account by a 'NASA photographic expert'. They alleged that in 'Building 8' of the Johnson Space Center, images of UFOs were 'regularly airbrushed out' from the high-resolution satellite imaging. McKinnon claims this encouraged him to investigate further. 'I logged on to NASA. They had huge, high-resolution images stored in their picture files. They had filtered and unfiltered, or processed and unprocessed, files.
Scroll down for more
Hacked: Computers in the Johnson Space Centre
'My dial-up connection was very slow trying to download one of these picture files. And as this was happening, I was able to briefly see one of these pictures.
'It was a silvery, cigar- shaped object. There were no visible seams or riveting. There was no reference to the size of the object and the picture was taken presumably by a satellite looking down on it. The object didn't look man-made. At my crowning moment, someone at NASA discovered what I was doing and I was disconnected.'
Hardly conclusive, as he now admits. But McKinnon was hooked. As he spent more time online hacking into American systems, marvelling at what he could see, the rest of his life took a distant second place.
Usually, he was careful to cover his tracks online, logging on only when it was night in America and there was little chance that anyone was legitimately using the system.
'I'd always be juggling different time zones,' he said. 'But there was one occasion when a network engineer saw me and actually questioned me.' He was challenged via e-mail, he says. 'The engineer said: "What are you doing?" which was a bit shocking. I told him I was from Military Computer Security, which he fully believed.'
McKinnon's hacking got more dangerous after the 9/11 attacks. In fact, on September 23, only 12 days after the Al Qaeda atrocities, he hacked for a third time into the Earl Naval Weapons Station in New Jersey.
The base is responsible for the munitions re-supplying of the U.S. Atlantic fleet. As a result, according to the U.S. Department of Justice, its network of 300 computers were shut down for a week. The authorities claim he altered and deleted vital files, rendering them inoperable.
'I would hope that this was not possible,' McKinnon says now.
The other side demurs. 'This was a grave intrusion into a vital military computer system at a time when we, as a nation, had to summon all of our defences against further attack,' Assistant U.S Attorney Scott S. Christie said in the statement.
Another time, he shut down 2,000 Army computers controlling the Military District of Washington, for 24 hours. But as he probed deeper into the U.S. systems, McKinnon was less careful, posting anti-war notes on the military sites, signed 'Solo'.
The hunt for him grew earnest. NASA alerted UK security services. After what McKinnon believes to have been several months' surveillance, the authorities moved in.
They took his computer, Tamsin's cousin's computer and several machines he was fixing for friends. They even took Tamsin, as her e-mail address was at the root of the hack.
'She said to me: "First, you screw us up financially, then you get us involved in an international incident",' he recalled last night.
He says of Tamsin now: 'I stay in touch with her through friends. She wishes me well, but I guess she's glad to be out of all this.'
After some consideration, the Crown Prosecution Service decided not to press charges against him. The hacking had been conducted against foreign computers.
But in 2005, after the latest extradition treaty had been passed, the Americans finally applied for his removal. And Home Secretary John Reid granted the request.
Last year, the High Court rejected McKinnon's appeal. Since his arrest he has lost a number of jobs and been kicked out of at least two rented flats because of his notoriety. He now lives on benefits and his bail conditions prevent him from accessing the web.
Today, he says he has now foregone hacking and smoking dope. It got him into too much trouble. He wants a new life.
His barrister, David Pannick, QC told the Lords' hearing this week that McKinnon had been 'threatened' by U.S. authorities, and that unless he agreed to plead guilty and to his extradition, he would be treated as a terrorist case.
This could mean a life sentence in a maximum security jail, rather than a couple of years in a standard prison.
Last night, McKinnon told me: 'I would gladly face trial in the UK now under the Misuse of Computers Act. That is only fair.
'Britain is the only country in the world that will extradite its own nationals without prima facie evidence. I am hoping that if they (the Americans) do get me, I will face a public court over there, with a civilian jury and not a military tribunal.
'We have received an unsigned diplomatic note that I will not go down that route, to Guantanamo. But the final decision is the President's, so I would not trust that promise as far as I could throw the Pentagon.'
His lawyer, Karen Todner, says: 'Gary is a bit quirky. What he did was wrong. But he was not politically motivated. He is just in way out of his depth and it's clear they want to make a terrible example of him.'
So, is Gary McKinnon a 'dangerous man' as the Americans allege? Or is he merely a reckless, X Files obsessive, who is now to be used as a warning to other hackers?
Last night, he sent me a disjointed text message. It ended: 'Can hack the Pentagon cant (sic) work my mobile.'
Armed with a keyboard and clad in a dressing-gown, he took on and damaged a superpower. He might now be about to pay the price.
Tamsin, a librarian, is the other tragic figure in this story. For several years, McKinnon was living with her in a flat in her aunt's house in Hillfield Avenue, Hornsey, North London.
One can only imagine Tamsin's frustration (she has not responded to my requests for an interview) as his hacking obsession took over.
In late 2000, McKinnon even gave up his paid job as a computer administrator to devote himself to cyberspace infiltration. Soon, he stopped washing, became noctural, ate rarely, smoked marijuana and spent all day in a dressing-gown.
Understandably, Tamsin dropped him - although, out of fondness or insanity, she continued to share the flat with him. She really should have got out then, because his night-time activities were now alerting some very serious people.
Between early 2000 and autumn 2001, McKinnon hacked into 97 computer systems, belonging to the U.S. Army, Navy, Air Force, Department of Defense and NASA.
He could do this because their security was unbelievably lax. 'It was so easy,' he says today. 'I found out that the U.S. military use the Windows computer software system. And having realised this, I assumed it would probably be an easy hack if they hadn't secured it properly.'
McKinnon's cyber crowbar consisted of several readily available programmes 'glued' together by one of his own.
The package searched for 'blank passwords' - computers whose passwords hadn't been changed from their default setting. It was no harder than climbing a neighbour's fence. Online, he could scan '65,000 machines in little more than eight minutes'.
Once he had found a loophole, he was into the host network.
'It wasn't very clever,' he has said. 'There were no lines of defence. And I wasn't alone in doing it. Often, I could see that there were a number of other foreign hackers present at any one time.' So what did he see there?
One example he gives was an account by a 'NASA photographic expert'. They alleged that in 'Building 8' of the Johnson Space Center, images of UFOs were 'regularly airbrushed out' from the high-resolution satellite imaging. McKinnon claims this encouraged him to investigate further. 'I logged on to NASA. They had huge, high-resolution images stored in their picture files. They had filtered and unfiltered, or processed and unprocessed, files.
Scroll down for more
Hacked: Computers in the Johnson Space Centre
'My dial-up connection was very slow trying to download one of these picture files. And as this was happening, I was able to briefly see one of these pictures.
'It was a silvery, cigar- shaped object. There were no visible seams or riveting. There was no reference to the size of the object and the picture was taken presumably by a satellite looking down on it. The object didn't look man-made. At my crowning moment, someone at NASA discovered what I was doing and I was disconnected.'
Hardly conclusive, as he now admits. But McKinnon was hooked. As he spent more time online hacking into American systems, marvelling at what he could see, the rest of his life took a distant second place.
Usually, he was careful to cover his tracks online, logging on only when it was night in America and there was little chance that anyone was legitimately using the system.
'I'd always be juggling different time zones,' he said. 'But there was one occasion when a network engineer saw me and actually questioned me.' He was challenged via e-mail, he says. 'The engineer said: "What are you doing?" which was a bit shocking. I told him I was from Military Computer Security, which he fully believed.'
McKinnon's hacking got more dangerous after the 9/11 attacks. In fact, on September 23, only 12 days after the Al Qaeda atrocities, he hacked for a third time into the Earl Naval Weapons Station in New Jersey.
The base is responsible for the munitions re-supplying of the U.S. Atlantic fleet. As a result, according to the U.S. Department of Justice, its network of 300 computers were shut down for a week. The authorities claim he altered and deleted vital files, rendering them inoperable.
'I would hope that this was not possible,' McKinnon says now.
The other side demurs. 'This was a grave intrusion into a vital military computer system at a time when we, as a nation, had to summon all of our defences against further attack,' Assistant U.S Attorney Scott S. Christie said in the statement.
Another time, he shut down 2,000 Army computers controlling the Military District of Washington, for 24 hours. But as he probed deeper into the U.S. systems, McKinnon was less careful, posting anti-war notes on the military sites, signed 'Solo'.
The hunt for him grew earnest. NASA alerted UK security services. After what McKinnon believes to have been several months' surveillance, the authorities moved in.
They took his computer, Tamsin's cousin's computer and several machines he was fixing for friends. They even took Tamsin, as her e-mail address was at the root of the hack.
'She said to me: "First, you screw us up financially, then you get us involved in an international incident",' he recalled last night.
He says of Tamsin now: 'I stay in touch with her through friends. She wishes me well, but I guess she's glad to be out of all this.'
After some consideration, the Crown Prosecution Service decided not to press charges against him. The hacking had been conducted against foreign computers.
But in 2005, after the latest extradition treaty had been passed, the Americans finally applied for his removal. And Home Secretary John Reid granted the request.
Last year, the High Court rejected McKinnon's appeal. Since his arrest he has lost a number of jobs and been kicked out of at least two rented flats because of his notoriety. He now lives on benefits and his bail conditions prevent him from accessing the web.
Today, he says he has now foregone hacking and smoking dope. It got him into too much trouble. He wants a new life.
His barrister, David Pannick, QC told the Lords' hearing this week that McKinnon had been 'threatened' by U.S. authorities, and that unless he agreed to plead guilty and to his extradition, he would be treated as a terrorist case.
This could mean a life sentence in a maximum security jail, rather than a couple of years in a standard prison.
Last night, McKinnon told me: 'I would gladly face trial in the UK now under the Misuse of Computers Act. That is only fair.
'Britain is the only country in the world that will extradite its own nationals without prima facie evidence. I am hoping that if they (the Americans) do get me, I will face a public court over there, with a civilian jury and not a military tribunal.
'We have received an unsigned diplomatic note that I will not go down that route, to Guantanamo. But the final decision is the President's, so I would not trust that promise as far as I could throw the Pentagon.'
His lawyer, Karen Todner, says: 'Gary is a bit quirky. What he did was wrong. But he was not politically motivated. He is just in way out of his depth and it's clear they want to make a terrible example of him.'
So, is Gary McKinnon a 'dangerous man' as the Americans allege? Or is he merely a reckless, X Files obsessive, who is now to be used as a warning to other hackers?
Last night, he sent me a disjointed text message. It ended: 'Can hack the Pentagon cant (sic) work my mobile.'
Armed with a keyboard and clad in a dressing-gown, he took on and damaged a superpower. He might now be about to pay the price.
