My PC has recently sprung a new problem on me that I can’t solve. I am by no means a noob but this has got me baffled!

I am using Windows Vista with Intel Core 2 Quad CPU

The problem is that my PC will turn it’s self off for no apparent reason… I can be doing my normal work at my PC and it just goes off, no "Windows is Shutting Down" message no logging off message, nothing, just turns it’s self off.

If I try to turn it back on it wont boot up, I need to wait a few minutes before I can turn it back on (this suggests to me that my PC turned off as a precaution as my processor was over heating but I don’t know to be honest)

It started to happen more often so I ran Windows task manager to see what could be pushing my CPU so hard… nothing obvious there but all 4 cores were running at 100% when my PC turned it’s self off again so I’m pretty sure my PC is turning off because of it but I don’t know what is causing my CPU to run at 100%

I’m running a lot of programs at the same time I must admit but this was never a problem for my PC (at most it would use 80% total CPU for the same amount of open applications and that really is at the very most!)

Needless to say I have ran a virus scan and nothing come up (NOD32 anti virus)

I have also used CCleaner… Run Reg Cure and Registry Cleaner, Tune Up Utilities 2009 and Malwarebytes’ Anti-Malware… none of them helped even a small bit.

Stupidly I deleted my restore point that was created before this problem started so thats not an option.

I appreciate any help as always.