i would suggest downloading the diagnostics software for your hdd from the manufacturer... this is just to rule out a possible hardware problem with the hdd.
The diagnostics app usually will run from a bootable floppy. Try that. If it still doesn't work - i would suggest reformat again. If the issue continues - there is definately a hardware issue somewhere.
We gotta go through the process of elimination. You could also try removing any other devices such as sound cards, trying another video card, modems etc.
if that doesn't make a difference, possibly a motherboard issue on your hands.