The vast majority of PC power supplies tie the motherboard and peripherals to earth ground through the black ground wires, but a few don't because their black wires are kept separate from earth ground inside the supply, except at high frequencies. With such supplies it's important for the motherboard, drives, and supply to all be grounded well to the computer case, and it's possible that a good paint job will prevent this. Old IBM or Mitsubishi desktop cases were masked off before painting to keep the metal bare around the screw holes, so I would remove the paint there or at least take ohm measurements to verify low-resistance grounding.