My Mom gave me one of her co-workers computers to fix up, but the first problem I run into is slightly stumping me: It's a HP Pavilion xt878, I plug the power supply in, a green LED on the PSU lights up. I push the power button, the green LED goes off, nothing else happens. As in, the computer is not turning on. And green LED turning off seems to me to be either a dead PSU or a misfiring in the wires causing the computer to turn off instead of turning on. I've tried several ways, unplugging the power, replugging and pressing the button again, and the only thing I've found is if I hold down the power button in the front (there is no power switch on the PSU) for a little bit the green LED comes on. So does anyone know anything that can tell me what the problem is? Please and thank you all for your help, whoever responds.

Failing power supplies are common and can cause your symptoms. They often partially work, yet the computer won't fully boot. Check your PS. See response 4 in this: http://www.computing.net/hardware/w...