|Sounds like a virus infection.|
You can try the following:
1. boot from the windows installation cd
2. on the "Welcome To Setup" screen, press Enter
3. accept the License Agreement. Windows will search for existing installations
4. select the windows xp installation, you want to repair and press the key R
5. now all necessary files will be copied to the hard drive
7. do not press any key, to let windows start from hard drive.