This problem seems fixed but I don't know why and it bugs me.
I built a new computer at the start of last month based on a core i7 930. I loaded Windows 7 64bit with no problems and left the system running overnight doing some stress tests. In the morning things were still fine so I loaded up some app's and used the computer for about 3 hours without trouble. The problem came when I tried a restart where the PC got as far as the starting windows screen then switched off. No BSOD no errors just off. At first I thought I had a hardware fault but after reloading windows the problem went away.
To cut a long story short I eventualy found that the problem only happened after installing Photo Shop Elements 8 and it happened every time. Later I discovered the same thing happened after installing my HP Scanjet G3110.
As I said at the start the problem is now gone but the only thing that changed was that I installed a printer on the system for the first time. Now when it gets to the point in the boot process where it would switch off it now checks the printer and carries on as normal. This bugs me as I can't see why installing a printer should have any effect and will the problem come back when I have important stuff loaded. Any ideas ?