Click start then run, type sfc /scannow then press enter to replace the system files, you need the XP CD.
Windows File Protection will show an on screen blue progress bar which will simply disappear when the scan is complete, reboot when it does
System File Checker gives an administrator the ability to scan all protected files to verify their versions. If System File Checker discovers that a protected file has been overwritten, it retrieves the correct version of the file from the cache folder %Systemroot%\System32\Dllcache or the Windows installation source files, and then replaces the incorrect file.
System File Checker also checks and repopulates the cache folder. You must be logged on as an administrator or as a member of the Administrators group to run System File Checker.
The System File Check is only concerned with replacing missing or corrupt system files, and does not affect installed software or data
If you do not have an XP CD then you can borrow a same version as was originally installed XP CD, if you downloaded SP2 then you need an SP1 XP CD