If you really do think it is the HDD, and it doesn't sound like it to me, have you got a spare computer to try it in?
I would suggest that you first open the case and as Simon advises check that the graphics card is fully seated, plus any other cards you might have. Check that all the cables are firmly home (but be careful that you don't bend the motherboard) and then restart the computer. If it still freezes, switch off and then restart in Safe Mode. If it freezes in Safe Mode, then my guess would be that your motherboard is either shorting, or is simply knackered.