If the computer you are having problems with is an emachines model that builder has a reputation for using power supplies that are a lot more likely to become defective and fail (often Bestec brand) - if the PS does fail, it's a lot more likely than average to damage something other than itself - in cases reported on this site, that's often the mboard.
ATX PSs are always powering ATX mboards in some places even when Windows is Shut Down or in Standby or Hibernate modes, so as long as the AC to the PS is on, the power to the mboard is not off.
Power supplies can turn on the computer by themselves if AC is being supplied to the computer/PS...
- and settings in the bios Setup for devices to "wake" the computer are favourable, such as if "wake on LAN" or simlar is on.
- sometimes just a power spike or surge will cause some PSs to turn on.
I have one that sometimes starts the computer by itself when I switch on the power bar it is connected to, despite the fact I am using an anti power spike/surge device - I suspect the power switch is creating electric or electronic noise that the PS reacts to like the pulse produced when you press the power button.
- there is a setting in most bioses where you can set it to turn on the computer after a power failure. "I unplugged the machine for about 30 seconds then plugged it back in. Of course it went off. " It sounds like that is turned off on that computer. Whether that is turned on or not, sometimes a power spike or surge will cause some PSs to turn on.
"There was a small storm last night but no power outages and I had my PC on, this one was off, during and yet it works fine (even while this one does not)."
" Note, I can honestly say there is no lightning damage"
You can't know that for sure.
The PS in one computer may be damaged by a power spike or surge yet another is not. The power does not have to fail in order for you to get a power spike or surge. If you have both computers and everything connected to AC that connects to them, and the telephone cable and/or coax cable for your modem, protected with a good quality protection device, you greatly lessen the chance the PS or other components will be damaged by a power spike or surge, but sometimes even that is not enough (e.g. if lightning strikes the power grid somewhere, even if you're not aware of that). If you do not have the computers etc. connected to a device that protects them from power spike or surge damage, the chances are much greater they will be damaged by such.
Check your PS.
See response 4 in this:
http://www.computing.net/hardware/w...
