Windows 7: Unexpected shutdown if system service "power" is running

so my computer started unexpectedly shutting down about 60 seconds after loading windows today. No BSOD, no error message, no warning, just a hard shutdown. It happened consistently every time I started the computer, unless I was in safe mode, then I had no issues. So I went into msconfig, and eventually tracked the problem down to a specific system service - "power". If that process is stopped, no hard shutdown. But that process seems to be responsible for Windows Audio Endpoint Builder, so I have no audio if it is stopped. Can anyone think of any reasons for this process to cause an unexpected shutdown? and how do I go about fixing it?

Sounds like a driver power state failure.
Turning on driver verifier may cause a bsod we can then read the minidump for and get some more direct answers. Anything right now would be pure guesswork.
Instructions for activating driver verifier follow
Driver Verifier - Enable and Disable
(Please read all instructions for driver verifier before using as it's possible it will make the machine behave badly and you need to know how to undo it.)

alright, followed the guide and turned on verifier, but I ended up with the same hard shutdown, still no BSOD or error message of any kind. also no DMP file in the minidump folder (that's C:\Windows\Minidump right?) not sure what to make of this. . .

that is not what I expected to be honest.
Hard shutdowns like that are usually an indication of a driver fault.
It could be hardware related as well, that's basically the reason I'm trying to get a dump file produced here.
You can try this too ...in order to try and get a minidump written.

To ensure minidumps are enabled:

Go to Start, in the Search Box type: sysdm.cpl, press Enter.
Under the Advanced tab, click on the Startup and Recovery Settings... button.
Ensure that Automatically restart is unchecked.
Under the Write Debugging Information header
Select Small memory dump (256 kB) in the dropdown box (the 256kb varies).
Ensure that the Small Dump Directory is listed as %systemroot%\Minidump.
OK your way out.
Reboot if changes have been made.

alright, still no luck getting a minidump after that last suggestion. But through trial and error, I found that I can make the unexpected shutdowns stop if hyper-threading is disabled in the BIOS; on the down side, that seems to make my computer only recognize two cores on my quad-core processor, instead of all four. I don't see how hyper-threading and this particular system process could be related.

Do you by chance know the make and model of your motherboard?

Asus P5N-D, CPU is a Core 2 Quad Q9550, and the BIOS is the latest version.

actually the bios was what I was driving at.
..and sadly was about my last thought as to a cause here.

I'm totally stumped. I even rolled back to a previous BIOS, but no difference.

my laptop automatically shut down after a few hours, and i have checked power setting and i didnt see any unusual thing !
please help me! tanx