Converting a standard ATX to work on a Compaq motherboard

Well, after studying a motherboard given to me by a customer of mine, and after swapping in multiple ATX power supplies. I noticed something. Even though the motherboard was a 20+4 pin like most standard ATX power supplies, this one had different voltages set in different plugs. UGH! Now I just have to design a way of converting a +5v to a -12v current :/