Computer restarting after proper shut down.

**Update: I found the issue. It was a motherboard setting. PME Event Wake Up was enabled. When I disabled it the computer stayed shut down. Is there anyway I can delete this thread?

To Whomever Can Help,

First, thanks for taking the time to read my post. I am having an issue with a homebuilt system exactly as stated above. The computer will not stay off. It always with no exceptions restarts after a proper error free shutdown. I built a virtually identical computer for my stepfather and have not had this issue with his system. They have the same Processor, Motherboard, Primary Disk Drive, PSU and Case. We utilize the same 8GB Mushkin Blackline in the first paired DIMM slots although, I have additional ram in the second slots, Mushkin Redline 4GB. I am at a loss. I've tried doing some searches online and inevitably I am presented with two solutions neither of which have worked. The suggestions I found online and have tried and failed are as follows:

1) In the registry, navigated to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon

Found the following key "PowerdownAfterShutdown"

Then, changed it to "1"

2) Control Panel> System and Security> System> Advanced System Settings> Startup and Recovery Settings> Unchecked Automatically Restart

A third suggestion was to look at the Event Viewer but I could not make heads or tails of what I was looking at.

Additional Information:
I seem to recall having this issue a year ago when I first put the system together but I beleive the issue dissappeared at some point early on; No explanation as to why. I performed a clean install on my system not 4 days ago and this issue was still present. This in addition to the things I've tried above on the software side leads me to believe it's a hardware issue and if I had to guess I would think it's a motherboard issue. The motherboard I have is a Gigabyte Z68-D2H-B3. I looked around in the bios and didn't see any settings that I thought would pertain to this issue. I did try changing some settings but ultimately nothing changed and I reverted back to the fail-safe defaults. Another thought just occured to me, could I have connected something wrong from the case leads?

This is the first system I've built and am competent but by no means an expert so please speak to me as if I'm ignorant it might save us time from having to ask you to clarify somehting I don't understand. Any help you can lend me will be greatly appreciated if you want/need any additional system information or just additional info in general let me know. Thank you all so much in advance for your help I look forward to your responses. Also, if there is a more proper place for this post please advise.

-REG

**edit: Came across a thread where the issue for the individual was the PSU. He had slightly different symptoms though. He could not prevent the restart from what I understand which, is not a problem for me. Nevertheless could my PSU be the culprit? It's an OCZ XStream-Pro 600W.