welcome to PCR
Go to Start, then to Run, and type in "SFC.EXE /SCANNOW" (without the quotes and with a space between the SFC.EXE and the /SCANNOW). Then press Enter.
You will need your XP installation CD. If you don't have one, you'll need to locate a directory on your system that's named "i386" (without the quotes). This directory may be on a hidden partition on your hard drive. If you don't have either, you need a good friend to loan you their CD
HTH