A supply losing the ability to regulate will be pulled down just at post time when the largest load other then any ocing is seen! That can be a real headache at times!
Disabling all startups in the msconfig while booted up in safe mode however is the usual advice for diagnosing device driver
and startup issues that can result in the dreaded BSODs(blue screens). The freeze ups here were being caused by errors on the drive likely due to a failed integration of SP1 into the iso used to create a usb install key for the recent clean install. SP1 had to be installed separately the first night to get that on.
Another low cost item to consider especially for any pc a year or older most wouldn't even think of is the lithium battery on the board. Once that gets weak you can be right in the middle of something and "crash" or "sudden unexpected restart" time!
rarename's problem however seems identical to what I was seeing a few day ago where once 7 booted up and you went to open a browser or explorer window everything froze up and you couldn't even move the mouse forcing a hard boot with the reset button. I got in there and clicked the Check Now button for C once I was able to move around once back at the desktop and after a good period of time 7 was suddenly running well with no freeze ups seen since.
There are a good number of possible causes regardless where you can never rule anything out until you look at each. The thing I would suggest rarename trying out would be to see if the system can be booted into the safe mode where the msconfig utility can be opened there to deselect everything in the Startup tab as well as hiding all the MS services to reduce the other 3rd party services in order to try booting a bare Windows normally.
If the desktop loads right up and nothing locks you will know then that this is not pointing at any specific hardware fault but strictly a software related issue of some type. The run of both memtest if not already seen to as well as scheduling the Check Disk tool would still be advised here however.