It should be a virus. Try going to a website and scanning for a virus as I stated above. If that doesn't work, try booting to safe mode, and restoring the PC using system restore.

If that doesn't work try typing:

sfc /scannow

and make sure you have your XP CD ready. That is the system file checker utility and it will find and fix any missing or corrupted files.

If that doesn't work, try uninstalling any third party applications. Start with one, reboot and see if the problem persists. If so, go to the next, the next etc..

If that doesn't work, it may be a time for a reformat..