Pick up a standard 12V relay. Wire the relay coil to your PSU 12V (yellow).
Then wire the 230V (110V?) from PSU power outlet (drill a hole on the side to let the wire go through) all the way to your relay switch.
Finally connect the other side of the switch to an internal plug (yep those standard wall-mounted plugs), connect the PSU black (GND) to the other connector of your internal plug.
Voila, connect the pump to the internal plug... When your ATX PSU powers up (thanx to your mobo) your pump is automagically turned on
