A temporary thing to do is turn off "automatically restart" by accomplishing a right click on My Computer, select Properties, then the Advanced tab. Find Startup and Recovery then click the Settings button and finally remove the check mark on automatically restart.

This won't prevent the computer from freezing but it might give you the opportunity to download and run memtest86. If it is memory, it should show up during that test. If you have multiple memory sticks, you can try one stick at a time to eliminate a bad stick.

I would also consider that the CPU could be overheating. Dust in heatsink veins can create a heating problem.