You probably have a proprietary power supply, one that was designed to be used with a specific motherboard (ie; Compaq). I had the opposite problem - a motherboard (Compaq) that needed a 24-pin PS. The pin-outs CAN be found, but even then, re-wiring the connector can be a lot of work, and it's possibly only a 180-235 watt power supply. Short version - it's not worth it.