I just bought an Antec 500W Neopower power supply. I put it in an older computer of my wifes and it ran for about 2 seconds and shut down. I've moved around the cables in the back and it didn't help. I tried unhooking the CD Rom drive and that got it to run for about 15 seconds but then it shut down. It is an old e-machine with an AMD processor. When I start it, the processor fan and fan in back of the comp all run. Any idea what the problem is?