If you are missing files of a Windows component, then you can run the System File Checker. Windows File Protection only monitors and maintains XP system components. If it is a 3rd party application, you may need to reinstall that application to restore the missing files.
Method #1:
To run Windows System File Checker:
Go to Start>> Run, type in: sfc /scannow [Enter]
Windows may require you to insert your XP CD.
Method #2:
You can also use System Restore to recover your system back to date before the files were missing. To activate System Restore:
Go to Start>> All Programs>> Accessories>> System Tools>> System Restore.