HP will not power up

I have a 511w 1.2 celeron w/ 128 mb ram. Two weeks ago the usb ports quit working. Now the computer will not power up. The power supply light on the back of the power supply comes on. I have tried different power cords, different wall receptacles still no luck.
I have pulled the power supply out and jumped out pins 4 and 6 , the power supply came on fan worked and the connectors had correct voltages.
I put another known good power supply in computer and it didnt power the computer up either.
i jumped the power on switch pins and still no start.