Right, so even though you unplug your power supply, the capacitors can still hold a charge, see if once you unplug the power supply unit the unit can still hold electricity. If you have the PSU unplugged, try and hit the power button and see if the CPU fan runs for a split second. This should always be dome before you work on a PC. This ensures that you are completely draining the capacitors of their potential charge. You should always use a strap as well.