Hard drive not registering. PLEASE HELP

I recently wiped my friends HD. It was an old HP and I downloaded all the correct drivers off hp.com for that specific model. I installed Windows XP and all the drivers for him and its ran fine, that is until I restarted. For some reason, BIOS is now not recognizing the HD anymore. I have no clue why. I went to the BIOS options menu and the HD is not even a selectable option to boot from. PLEASE HELP me fix this problem.

BTW, hp.com had a driver for a BIOS update. I noticed after I installed that update and rebooted, it sopped recognizing it. Is there a simple solution to this? I know just a little bit about computers to get me by, but I'm willing to take any REAL advice. I'm not about to dunk my friend's PC in water and see if it still works.

Thanks all.

>>3
1. Exactly. Look it up in your motherboards' manual, or, if it didn't come with one, try to see which brand and model is it (look for it printed in the mobo near a border, the center, or the larger chips). Or try to find a couple of pins labeled something like "CMOS RESET". Turn on the computer with a jumper in them, then turn off, and remove jumper.

2. How do I reflashed BIOS? ;) You *need* to find your mobo's brand and model for this, then go to their website, and get the latest BIOS for that *exact* motherboard. Then proceed to follow their instructions, not mine. In case of doubt, don't flash.