The best way of finding a person who knows about fixing pcs (IME) is word of mouth. A bit like any trade really.
It may be worth asking PC World for information about getting hold of a restore disk specific to the machine they sold you. Persevere with this. You need to speak to the right person and tell them you expect help with a product they sold you. 5 years really isn't that long (many people would have you believe differently in terms of it's not an up-to-date state-of-the-art machine, but that's not the whole story; manufacturers often have resources available for purchase / free download for older models of machines). With the correct restore disk, as long as you back up all your data first, you can stand a far better chance of giving your pc the correct drivers it needs and reverting it to the state it was when you first purchased it.
This assumes you haven't modified the hardware too much in the intervening years. Also, if you do go down the restore disk route, be prepared for Windows Update wanting to download Service Pack 3 for XP and a bunch of other stuff after you do!