Have you found a PSU that will work in your Compaq yet Melkor? Compaq has a habit of using Propriatary equipment. Including the powrsupply. In my last compaq it had what they called a mini ATX motherboard. But it required Compaq's version of an AT power supply. In otherwords make sure what you buy will actually work with the compaq motherboard. THe connectors may be completely different or have different pin assignments than a standard ATX power supply.