Do memtest86+ I think... or if I remember the windows disk has a built in memory diagnostic...
Also there's programs like OCT
that let you stress test your processor, memory, and videocard. And if you use it, you should get a freeze a lot quicker to tell you what the problem is.
At the current state it could be anything from a hardware or drivers.
Also, if you could list what your PC is. It might be easier for us, or a different forum to diagnose the problem. As a mobo, or videocard may have a known issue.