Get a windows XP cd and boot from cd, if you are able to load files and get to the install screen, then its a software issue, possibly a virus that corrupted your registry. All you need to do is reinstall windows again. You can also Use Ubuntu to perform the test its free and reliable, download it from its web site and make a bootable cd. Instructions are in the website.
If you cannot boot with a boot CD (win or ubuntu) then you may have a hardware issue, a lot of times its a bad hadr drive that requires a new HDD and OS system install.