Dell power supply question?

I recently picked up a Dell OptiPlex GX150 Series Desktop/PC(small form factor). When power is turned on nothing appears on the screen. PSU and CPU fans are on but drives don't boot up or sound like they are. I tested with a Aspire Power Supply tester when attached to connector all voltages are showing but the -5 volts. Could this be the culprit or could I be looking at a bad board or video?