OK, let's see if a repair will fix this up.
Try this first:
Go to Start->Run and type in sfc /scannow and hit OK. Let it scan. If it finds any files missing/corrupted, it may ask for the Windows CD.
If that doesn't fix it, restart your computer and insert your Windows XP CD. Boot from the CD. Let it load. At the blue screen hit Enter key to enter setup. Then choose to repair Windows (hit R key). Follow the instructions there.