My computer has been working with no problems until now. Out of the blue, it will no longer turn on. I have not moved\touched\changed anything. It just decided not to turn on now. I think the PSU is good, because I disconnected it from the motherboard and shorted out the green wire with a black ground wire, and all the devices powered up (cd-rom, hard drive, fans). But when I have it plugged into the motherboard, I push the button and most of the time it doesn't do anything. However, sometimes when I push the button, it acts like it wants to come on, as the fans and hard drives turn on just for a split second then shut off. But this only happens randomly and not too often. Most of the time I push the power button and it does nothing. I have also tried to see if it was the power switch causing the problem, by disconnecting the power switch cable from the motherboard, and shorting out the two pins on the mobo with a screwdriver to see if I could get it to come on, and nothing happens. Is there anything I'm missing? What else could possibly be the problem if it's not the motherboard? Could it be the CPU? I also tried disconnecting devices (hard drives, etc) and removing expansion cards, but no luck.

The power supply needs to supply several voltages. When you power on, the first thing that happens is the motherboard checks all the voltages. If any are missing or out of tolerance, the motherboard activates a line that tells the power supply to shut back down. To really check to see if the power supply is bad, you will have to swap it out or purchase a power supply tester.

Unfortunately I don't have another power supply to test with. And I don't want to have to shell out money to test it, especially if it is not the problem. I forgot to mention that when the power supply switch on the back is switched on, a couple lights on the motherboard light up. One being a green light (power light I guess?), and the others being for the LAN connection. Could the motherboard still be bad though?

I think it's your power supply. Just because the power supply comes on when shorting the 2 wires doesn't mean that it's good. Power supplies usually fail under a load. When you short the wires, the power supply is not under a load. I'd try swapping the power supply, but be prepared because the power supply may have damaged the motherboard. I've seen this happen way too often. WinSimple Software

I went and bought a new power supply, and low and behold it works!. Thanks for your suggestions, as I wouldn't have thought it would be the power supply based on these conditions.

Cool! WinSimple Software