A repair install may fix the problem. Understand that all the patches will have to be re-installed after this is over.
Pop in the XP cd - press a key to boot from it when prompted (you have 3 seconds).
When you come to the first screen offering a Repair -DO NOT PRESS "r". Hit Enter and you will get to another screen offering to repair an existing Windows installation. Now you choose "r". Only XP is overwritten; all your other stuff is untouched.