BIOS has nothing to do with it when a computer doesn't boot. It could be that the Window might be corrupted. Try to press F8 when power your computer and go into "Safe Mode" or "Boot from last configuration that work". When in safe mode than you can go to system restore. Hope this help.
I also had this issue on a desktop that someone gave to me that they were throwing away (they said it couldn't be restored due to viruses and registry issues). I tried everything listed above and nothing worked, so what I ended up doing was creating a KillDisk boot disk and wiping the c drive. After that just reinstall XP.
A remote possibility, I admit, but it may be possible your master boot record has been corrupted. It is easily fixed as long as the drive itself is not faulty. Download and create a dos boot disk from http://www.bootdisk.com/bootdisk.htm then boot into that disk and run the command