All my full screen games freeze

I got Windows 7 64bit NVIDIA GeForce 9800 GTX+, Intel(R) Core(TM)2 Quad CPU Q9300 @ 2.50GHz (4 CPUs), ~2.5GHz 4096MB RAM 2 TB of Harddrive space 700 W power supply Why do all my games freeze? I've played Battlefield 2, Day Of Defeat: Source, Street Fight 4 amd Atlantica Online. They all freeze after 5-10 minutes of game time. And I can't do anything until i reset my computer. What is going on? My computer isn't overheating, and all my drivers are upgraded...

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