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.

Thanks for the info. But let me get this right.

1. Clear the CMOS. Should I be looking for a switch of some sort next to the battery or chip?

2. Reflash the BIOS. How do I do that?

And I only have the updated BIOS driver that's on the hp.com site.

Thanks again for the help.