Do you have an XP disc? If so have you tried booting from that into repair mode?

If worst comes to worst, and assuming you have backed up your data and it is a brand-name computer, you can power it down, disconnect all the peripherals (leave just keyboard, mouse and monitor, no internet connection either) and then boot it into system restore/reset (the process for that depends on the manufacturer, google will help you find it for yours). That will use an image on a hidden sector to completely overwrite your hard drive and reset your computer to the way it was when you bought it. You'll have to then install good AV software and spend some time on Microsoft's site getting patches, service packs and other updates, after which you can reinstall all of your programs and, finally, restore your data.

Good luck.