Click F8 at boot ... click Last Known Good Configuration. What will happen is ... Windows will be reset to all the basic control settings, which might get the computer restarted again, but you will still be infected, if infected. So if this works, boot up, and then you need to scan with a bunch of different scanners (preferably boot into safe mode to do this).
If that fails, try building a bunch of Virus scanning boot disks. There is a good website
here that shows you how-to. Check out the other disks further down the page.
Your best bet, without having the XP cd, is to build UBCD4WIN
here If you have decent ability at removing malware/ fixing an OS ... then this should get you going again. It has all the tools needed to sort most problems out - but it depends on your knowledge level. It takes ages to build too, so skip if you can't be bothered doing all that schizz
Good luck.