Hmm... sounds like a crappy situation. You can try to run registry mechanic than use windows check disk program and see what happens next.
If that fails you can always use windows recovery console to solve the problem.
Check these links may be you will find what you need there. Good luck
http://support.microsoft.com/kb/314058/
http://www.webtree.ca/windowsxp/repair_xp.htm
http://www.michaelstevenstech.com/XPrepairinstall.htm
http://www.winxpfix.com/page18.htm