What could it be?

So, you've done everything you can do. It's still not working. One thing you should remember is the bios jumper. All you do is find the jumper that says "Cmos clr" next to it, pull it out of the two pins it's attached to, attach to the next to pins directly next to it for a few seconds, and put it back. This resets your bios and cmos to the original defaults. This will also mess up your time. I've gotten 3 computers to boot up XP when they couldn't before by doing this simple step.