More answers!
Does the computer boot with anything, like a (shudder) msdos boot floppy?
-nope
When you say the bios is ok, what does it say you have on your ide ports? Will it auto detect correctly? How much ram does it think you have? Did you flash it recently?
-It auto-detects correctly, 128 megs, and no never flashed it
Did you have any unusual problems prior to upgrading, like programs suddenly acting wierd, or the system slowing down over time?
-No, I don't think so...
I don't think it's hardware because It's been long since I made modifications to my comp (swapping hd or anything)