Have you been given the option to go into Safe Mode? When it boots up? My best idea would be to boot it in Safe Mode (if I doesn't give you the choice, then switch it on and off and on and off a couple of times while its booting up - then it'll give you the choice! and see what happens).
If that doesn't work; doesn't bring up Safe Mode or doesn't work in Safe Mode then come back and tell us what happened!
Do you have a Windows XP Install CD?
A Knight is sworn to Honour. His heart knows only Virtue. His blade defends the helpless. His might upholds the Weak. His word speaks only truth. His wrath undoes the Wicked.