Computer will power up for three seconds then shut...

I have a compac presario 50000 desktop computer ,msi 6930 ver 30 b motherboard ,amd socket a . It will try to power up then stop within say 2 to 3 seconds of hitting the on button ,just goes dead power supply light is on. I tried switching memory sticks with a known good stick. I have an extra socket a cpu and both start to get hot for the 2 seconds the machine is on . Hard drive is fine . Putting the cpu cooler on makes no difference. Voltages seem to check fine on the power supply. What do you think bad mobo/ or capasitor or capasitors on mobo. Thanks again Fixya, Eric