I had bought the build two months ago. (Specs in my signature) After some days this problem appeared. The machine was automatically turning on after shutting it down from Windows 7 32bit. After 15 days, the motherboard died suddenly while playing a game on it. Intel replaced that mptherboard with another one in working condition in 25 days. I got it back this saturday and bought a Corsair CX430 PSU and a CM Case Fan. It all went smooth till yesterday when it was on for 8hours straight and my mother asked me that the PC is turning on automatically after shutting it down. Ohh, I did some BIOS fan related tweaks yesterday, does that matter? Only changed the CPU fan speed.
The coolermaster case fan has a 3pin connector and my mobo has a 4pin one. The BIOS recognised the fan as rear fan until I changed the setting from outlet fan to inlet fan and its not recognising the fan at all now.
Next find a bootable hardware diagnostic and run it. Overnight.
Next download some temp sensing software (realtemp, etc.) and see how hot your CPU and video are getting. (I didn't see video in your spec, maybe you are using integrated vid? If so once system is stable look into getting a video card).
Run windows update a few times, look at the optional downloads and see if windows can spot any device drivers you are missing. You may have picked up something bad during a new windows install. Check device manager and delete any device that has problems and let windows pick it up again on the next boot.
I just saw that that option was already not selected.
Idle CPU Temp. : 35C
Load CPU Temp. : 70C
Tweaked fan settings from BIOS.
The thing I'm complaining about is that after getting shut down completely (no lights) the machine turns on after 8-9 seconds. That's what pissing me. Could it be due to a short circuit between the motherboard and the case? or maybe the On/Off switch of the case.
pull the wire to the power button. The power button is an intermittent switch, it turns the pc on by momentarily shorting. Maybe the switch is bad.
Once the wire is off use this procedure to fire up PC "The power switch on the front of the case is just a momentary contact switch. The motherboard always has a little bit of power, and pushing the contact switch simply completes a circuit between two little prongs on the motherboard that signals the power supply to start supplying full juice to the system. When you open the case and trace the wires from the backside of the on/off switch down to the motherboard, you can pull the those two wires off the motherboard and touch the two prongs with a small screw driver (be careful poking around - touch only the two prongs that were underneath the wires from the switch). The system should come to life. " http://forums.cnet.com/7723-6122_102-130540.html
Once the PC boots up, shutdown normally. The switch is not connected. If the pc stays powered down, then strongly suspect the switch or a short in the wire to the switch (pinched under mb for example)
Oh, one more thing, check to see if 'WAKE ON LAN' or equivalent is set. Some PCs can boot when the ethernet adapter gets a packet targeted for the PC. Make sure this is not set or simply remove the network cable before shutting down. Since this is so simple to test you might want to do this before testing the switch. See http://en.wikipedia.org/wiki/Wake-on-LAN
Before turning the PC on today, I did one thing.
I just turned on the wall electricity switch and waited for atleast 15-20 seconds. The Mouse was getting power (lights were on in it) but the PC did not boot. So, I think the case switch isn't faulty. If so, it would turn the PC on after getting power, no matter after a shutdown or before the first start.