if you can find your xp disk then place it in your CD ROM drive and follow the instructions below
Start > Run > cmd then type
sfc /scannow (note the space)
let this run untill you get a message saying it is complete.
SFC - system file checker, retrieves the correct version of the file from %Systemroot%\System32\Dllcache or the Windows installation source files, and then replaces the incorrect file.
If you want to see what was replaced, right click My Computer > manage, expand event viewer > system.