You could try a system restore from a point before where this happened, or run System File Checker
. If you have an XP CD, it should be in the drive. Go to Start -> Run and type:
. . . and hit enter. In the DOS emulator, type:
. . . at the prompt. Let the checker run to see if it finds and corrects any errors. If the DLLcache folder is intact, it will replace any corrupted files, including those that have been replaced from Windows Update.