Microsoft will still be updating MSE for the next year. As Mr Haematocrit says, the major issue is that bugs in XP will no longer be fixed after next week. Over time, XP will become less and less secure as more exploits and weaknesses are found, but not patched. Your pc will probably be reasonably safe for a few months at least, if you keep the browser up to date, don't visit any dodgy sites, run something like the NoScript plugin for Firefox (reduces the chances of exploits via questionable javascript being run on your internet browser) and use a hardware firewall: the router your ISP supplied you with will probably be a reasonable hardware firewall.
Your other options are:
1. Disconnect your XP computer from the internet and use it stand alone without any network connections. I have some data analysis software that only runs on XP and that's exactly what I'll be doing: the computer it's on won't get linked to any network.
2. As discussed above, install Windows 7 on your computer, then install a "virtualised" copy of XP on that. XP will effectively run within the Windows 7 environment. You can configure things so that XP cannot connect to the internet so will be more secure, and you'll still be able to conveniently use your copy of AutoCad. This has the disadvantage that you'll need to fork out for a copy of Windows 7 and quite possibly spend more money to upgrade your pc to run Windows 7 sufficiently fast (you may need a new hard disk and extra memory).
3. Convert your computer to dual boot. XP will live on one partition of your computer's hard disk, and never connect to the internet and another operating system on another partition. That other operating system can be anything you choose: Windows 7 or 8 or (the zero cost option) Linux, but you can use that for email and browsing safe in the knowledge that it'll be more secure than XP. That's what I'd do, run XP and Linux, but that's because I'm mean (so I'm told

) and already have a laptop already set up in this way...
For the last two options, I'd get in a friendly geek to set it up for you, if you're not comfortable doing it yourself: you can offer them beer, prosecco ...or just threaten to banish them from the FNR if they don't!