Do you have a copy of the Windows XP CD? If so put it into the CD-ROM drive, make sure that in the system BIOS your computer is set to boot from the CD-ROM Drive before Hard Drive C. You will get a screen saying something similar to: Press any key to boot from CD...
Press any key, and it will take a few minutes to load all of the files. It will then ask you to select repair by pressing R, press R. Have it restore all of the files, then re-boot after the repair, this should fix the problem.
If you don't have the CD, download the file, and copy it to a floppy disk, along with a copy of a Windows XP boot disk, then copy the file to the correct destination and try again.
P.S. I hope this helps