Alright this is a little long, but PLEASE read it through. I'm really in a bind and none of my technician friends and colleagues could help. Ok, so I'm fixing my neighbour's computer. I tried to fix it over at their place, but it had a pile of software issues so I told them I'd take it home and work on it when I get a chance. So I take it home, open it up, blow out the dust with canned air, close it up, connect it, and hit the power button:... ... Nothing. - A quick flash of the LEDs and a momentary attempt by the fans to start, but that's it. Then every time I try to start after that I don't even get that much - no response. So here's the troubleshooting I went through: ("Ok probably a power problem, let's test a few things.") - Wall outlet fine - Power cable fine, tried 3, all good - Power supply tests fine - powers on when started by itself, and measures fine with voltmeter ("Ok, so the power is fine, maybe the switch is bad.") - Switch fine - it seems ok and I bypassed it by shorting the mobo jumper pins briefly, with the same result ("Ok, fine, 'Divide and Conquer' - let's pull everything unecessary off and try to narrow down the problem") - Unplugged and disconnected all components. (IDE&Floppy cables, RAM, Video Card, NIC, CMOS battery and power to drives) Only thing connected was CPU, and Power Supply. "WOW! it POSTs, beeping to whine that everything is missing. OK so I shut it down and gradually start to plug everything back in (starting with the ram, battery, etc). Start it back up - CRAP! nothing! Sure, alright, lets start a little slower. -Unplug it all again, hit power button - no dice, nothing. Then it worked. After fiddling and randomly trying some things, I realized that it only starts up after I have unplugged the big molex Power connector from the motherboard and plugged it back in. Only then, after it has been reconnected, will it start up (and boot just fine mind you). Unfortunately, as soon as I shut it down it won't start up again. If you think you can contribute anything, no matter how much of a longshot it might be, I'd very much appreciate the help, don't hesitate to post! Thanks in advance!!!

Sorry man, I have know idea, this is kind of stuff that’s been happening to me lately. It's almost kind of creepy. Well, maybe I do know. This kind of sounds like the power supply, even though you tested it and all. But you tested it with no load on it, and when you put a load on it.. Well you know what it's doing. Anyway try another power supply, maybe from your computer. It’s a long shot, but it just might work.

I would also suspect the PSU - they usually go suddenly, but I have had a couple of instances as you describe - first time I made the costly mistake of immediately suspecting the MOBO - bought a new one, plugged in, switched on, at which point PSU went completely & took new MOBO with it! I now keep an expensive, powerful PSU as one of the 1st tests I do on a misbehaving system.

Yeah, it was the PSU. I switched it with a known good one and it works fine. Thanks for everyone's help!