I have an Intel Core 2 Quad with 4 GB RAM and NVIDIA GeForce 9600 on Windows 7 Ultimate. Lately when I'm playing games the monitor would go off and show the "digital", "analog" like you just turned it on without the CPU. However, the CPU is still running. The speakers would also put out stuck sounds of the game. This problem didn't occurred until I played Civilization V and Starcraft 2, and now all my games have the same problem, except those that uses little graphics. I have tried reinstalling and formatting my windows, but the same thing happens when I play World of Warcraft, after 2 hours of gameplay. Can someone help? I have updated all my drivers to the latest version.
My best bet would be your videocard is overheating which causes your system to crash. Make sure the fan on it's cooler is running, that overall airflow in your computers housing is in order and that nothing is clogged with dust / hair or anything.
Could be memory related so you can check that with memtest. You can get that at www.memtest86.com
That usualy results in more random occurances and mostly bluescreening instead of freezing with a black screen.
I'm still leaning towards a heat problem with your videocard. I've had it myself due to overclocking. If you have another videocard you can check if the problem occurs with it or not. If it doesn't you know what the culprit is.