Foremost get Windows XP CD/DVD into the CD/DVD Drive
Step:1 - Click on Start
Step:2 - Click on Run
Step3 - then type this command :
sfc /scannow
Step:4 - Then click on OK.
Wait until the process is complete
This will check all your Windows system files to see if they have been edited or changed.
If they have, it should be able to replace them with backups, but if it can not find them on your hard drive so it will be necessary to copy from your Windows XP CD or DVD.