Bad PSU?

I have a Dell Dimension 9150. The other day when I tried to turn the computer on the power button was flashing a yellow light. I call tech support, they said it was either a bad PSU or Mother Board. I got a a DMM and tried to measure voltages. There is only one on the green line going to the mother board. I tried connecting that one to ground to get the PSU to start, nothing happened. I only get 3.8V on the green wire. Any suggestions?