It's hard to think of a circumstance where it would need reinstalling so why do you think it's corrupt or missing? If you needed to do it, you need to go Start then Run and in the open box, type sfc /scannow but you'll need your XP CD in the CD ROM in case the System File Checker wants to load new files from it. Post back if SFC command doesn't work - it may not if you have XP Home.