As the four Diagnostic lights are off and there are no 'Beeps' and if you are comfortable working around computers, you could try the following:

Note: The only 100% method of testing a PSU, is to install a known working power supply.

Unplug the cord from the power supply, hold the power button in for about 15/20 seconds, open the case, unplug the 24-pin power connector from the motherboard and jump the Green wire to one of the Black wires, reconnect the power cord and power the system on, if power supply's fan and the hard drive run, then the PSU should be good.

Note: Do not remove any wires from the plug, use a small piece of wire or a paper clip as a jumper.

Power supply checks out and the system still does not work, again remove the power cord, hold the power button in for several seconds to discharge the residue power, reconnect the 24-pin connector to the motherboard.

Remove all the PCI cards, the video card [if applicable], memory, all peripherals, disconnect the data and power cables to all the drives, check that the front panel cable is connected to motherboard, with nothing else connected to the system, reconnect the power cable and power the system on.

If you do not get any beep codes and/or there is no change in the diagnostic lights, it would appear that the motherboard and/or the processor has died.

Note Processors rarely fail, motherboards fail more often.

The only method I know to check the processor, is to install it a compatible working PC, or use a known working processor.

Bev.