My older E-Machine will not power on?

We have an older E-Machine (W3080) (MSI KM4M motherboard) that will not power on. The system was operating normally, then just powered down. At first check, when hitting the power on, there seemed to be no response (no post beeps/etc). I opened up the case and noticed that when you hit the power switch, the cpu fan would attempt to spinup, but then stop (appr. 1/3 of a rotation). First umption was the ps had gone bad. Before pulling the power supply out however, I disconnected all the drives and connections and cut the power on out of curiousity and the power supply powered up normally. Reconnected the 20pin ATX power cord and the 4-pin 12v ATX power cord at which time, the same symptons returned. I disconnected the 4-pin 12V ATX cord and the power supply powered up and the cpu fan operated normally as well. However there was no post since the cpu itself had no power. Am I correct in uming that something went bad in the motherboard, or would it more likely be something in the cpu?