Is this either a PCChips or ECS motherboard with an SiS 735 chipset? I had the exact problem you describe.

I bought a new hard drive. The computer began acting funny a week later. It went downhill. I plugged the HD into my Dad's computer and it had no problems (Identical Motherboard). When I put it back into my computer, it was worse yet. It couldn't find ANY boot devices.

I ordered a new motherboard with an nForce2 chipset from NewEgg. Two days before it arrived, my old motherboard started working again.

I put the new one in anyway, though, for reliability's sake. It's faster, too.

BTW, AMD Athlon 1.0Ghz, 256MB DDR.