Try booting into safe mode and using the System File Checker
Start > Run > cmd
You'll need your XP cd
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.
System File Checker (sfc)
Scans and verifies the versions of all protected system files after you restart your computer. Repair Install Repair XP Install