As no one has mentioned this yet:

Do you have any bootable CDs or DVDs (like a Windows installation disk)?

If yes, try booting the PC from it. I recommend using the one-time boot order hotkey. As I don't know what your system is, I can't check what the hotkey is.

If you can boot from the disk, you have a Windows problem. If not, you have a hardware problem, and no amount of messing about in Windows will fix it.