Put your XP disc in your CD drive and exit from the setup program when it appears.
Type cmd in the box and press OK.
At the DOS prompt, type sfc /scannow (and press return).
Windows is not real smart about this. You may be prompted to insert your disc several times. Just keep pressing retry. This process can take some time, so be patient. When it has finished reboot your machine.
If you installed a program that over-wrote this system file, it will not function correctly after this procedure and will need to be re-installed.