Vista Multiple random Unexplained Shutdown

G'day,
My vista machine with all the latest service packs and updates has been randomly shutting down now for the last couple of weeks.
I did not install anything new, and have tried running antivirus and Malware programs, as well as HDD tests, and memory tests, to confirm there is nothing wrong there.

There is no specific time it stays stable, could be 10 mins, or could be an hour. Event Viewer shows Eventd ID 6008
"The previous system shutdown at 11:01:09 PM on 12/09/2011 was unexpected."

No Blue Screen, just black and shuts down. Looks like it loses power, but the following startup is normal.

I should add that every time I try to run a full scan, it never makes it through without shutting down. Either with Avast, or Malware Bytes.

I tried running the machine in safe mode to see if it was something to do with another program, but it still managed to shutdown all by itself in safe mode.

I have now tried uninstalling Service Pack 2, and will re-install to see if that helps.

Any help would be appreciated.

Test in safe mode. If not problem a clean boot will be give answer.
How to troubleshoot a problem by performing a clean boot in Windows Vista or in Windows 7
Download and make a full scan with malwarebytes. If you did not do so already.Its one of the best.

It does automatically shutdown, both in Safe Mode, and also in clean boot. I have not managed to get a full MalwareBytes Scan yet before it shuts down. The tough part is it sometimes lasts a few hours without shutdown and other times just a few minutes.

Just when I think I'm getting somewhere, it shuts down on me.

Sounds like it might be hardware
Have you cleaned all the dust from the inside around the fan especially?
Might, also be a BSOD see if there is any information here and post
http://www.vistax64.com/crashes-debugging/282419-blue-screen-death-bsod-posting-instructions.html
May, also, be any of these
http://www.computerhope.com/issues/ch000689.htm

Yes. Unfortunately I have cleaned the dust from all the inputs. I even had a a cpu temp program running (CPUThermometer), plus a HDD health program (HD Tune 2.55) running at one stage when it shutdown. CPU Temp was about 62DegC in each core. HDD temp only about 42DegC.

My HDD claims no errors. Malware Bytes No Errors, and Avast boot time Scan No errors.

Do you know of any other monitoring program I could try that may give me a reason. Because I don't get a BSOD, and the event log seems to think that it was a unexpected shutdown.

Everything tells me it shutdown, but nothing tells me why. When I was using the computer at the time of shutdown, all I saw was everything normal, then a jump in CPU Load and the screen went black. Then it was all gone. The startup was a normal boot after that. (ie no black screen offering me startup options)

Any ideas?

Can be so many reason, yours seems to point toward harware (happened in safe mode)
Give this a try

CPU Stress Test Hardware - Stress Test With Prime95 - Windows 7 Forums