Temps or video driver problems will usually turn out to be the cause unless a hardware fault like memory is being seen. You should examine the cpu as well as other temps.
Memtest is known to report errors when no faults are seen with the memory installed while still being the preferred stress tester. Which version of Windows are you running? XP? That's known for freeze ups when the installation has been on a long time.
Look over the list of startup items in case a 3rd party program is the source as well.
The first install of 7 here was getting a little buggy along with the XP counterpart on the same drive. I just finished setting a second copy of 7 with XP on another drive plugged back onto the system just to see each have a working copy. The second XP however is Pro not Home like the first.
There were a few times here on the two latest where Windows would freeze or appear to freeze since something was taking a bit too long in the background. One method for finding the source of a problem would be disabling a few things at a time in the msconfig to see if one of the startup items is the reason.