Sounds like a bad power supply to me. An existing installation on the drive should not cause the computer to shut down, and existing and incompatible installation usually results in the computer locking up when windows loads or the PC going into a reboot loop. You can very easily rule out the hard drive by simply disconnecting it and leaving ONLY 1 stick of ram, the processor and video card.
Also, make sure the bios reset jumper on the board is in the correct position and not in the reset position. I've come across a few boards where the jumper was on the wrong pins and the PC would not boot.