OK, I accept we are being a bit cute...
Can we assume you have up-do-date backups? If this is so (and by God it should be if you're using Windows) then simply reformat the hard drive and re-install Windows (preferable a version with SP2, but failing that get someone to download the Network installer of SP2 so you can update Windows without connecting to the Internet). Now reinstall your applications from the orginal CDs, and copy back your data from your backups. Then go out and buy some up-to-date virus protection software, and install it. Ditto for anti-adware, anti-spyware, etc. Finally, download and use Firefox, and cast MS Internet Explorer into the outer darkness it so richly deserves. Well, maybe leave it on the hard drive, for those MSIE-only sites like bloody Microsoft's support and updater pages.
Easy as pie.
I learned long ago that any computer problem that cannot be solved within an hour is usually best solved by reformatting the hard drive and then restoring from my backups.
If you don't have backups, then things become more complex. Do you have the option of booting up from another device, say an external hard drive? Or an iPod? The best thing would be to do that, copy across anything you haven't backed up (documents and whatnot) and _then_ erase the hard drive.
Possibly there is some anti-virus software that will do the trick. You should have at least one anti-virus program in the CD bundle that came with your PC. But again, your problem is running the PC for long enough for you to be able to run that program. Again, you really want to boot from an external drive.
At least with my Macs, I make a point to have a plug-and-play external device (i.e., an iPod) onto which I have system software and repair utilities. That way, if something goes wrong, I can at least boot up the machine and start some basic repairs. Ultimately though, your only real strategy is to rely on backups, so that when things do go pear-shaped, it doesn't matter (much).
Anyway, if your car was as unreliable as a Windows PC, you would have taken it back to the dealer a long time ago. Can you imagine a car that crashed once a week? Got damaged by things it picked up invisibly along the roads it was designed to drive on? That required you to upgrade the engine every year just to stop the thing exploding in your face? That allowed nefarious people you couldn't see to suddenly take control of the steering wheel and gear-stick? We're a lot more tolerant of design flaws in computers than we are with cars, and we shouldn't be.
Cheers,
Neale