Click Start > Run and type sfc /scannow and the click OK.
Note the space between the c and the /
You may need your Windows XP CD so have it ready.
If you have Service Pack 2 (SP2) or SP3 installed, you will need the SP2 or SP3 version of the version of the CD. This can be done with a borrowed CD, if you don't have one.
Allow the scan to run and when completed, reboot the system.