Why do my games keep randomly freezing?

Battlefield - 6/7 times it froze. Everytime it did I had to shutdown the PC with the power button. Each time its different, all but one time was when I engaged in battle, the one time it wasnt was when I hit a spawn point.

Thief - froze 1/1 times I played. I had gotten almost to the 3rd level when it froze. I didnt save since the begining of the level so I had no reason to play it again cause I was so mad.

Oblivion - Froze 3/4 times, the 3 times were random. One time was when I hit done when making a character. The fourth time didnt have time to freeze cause it was only about 2 minutes of play...I had to test my computers strength.

Call Of Duty expansion - 1/1 It froze when I was running to a fox hole...I went the wrong way, something hit me, and it froze.

I tried other games, but nothing really had much time to freeze. Just testing the highest graphics I could...

AMD Athlon 64 X2 5200
M2N-E ASUS motherboard
2 gigs corsair dual channel ram
7900 256mb Nvidia

I downloaded all the updates, did defrags, virus scans...everything...and I am completely out of ideas.

No just games. I have the Corsair 520watt...I think its 520...but I had a problem earlier. When Oblivion crashed, I couldn't do anything on the desktop. It said I didnt have adminastrative control or something. I couldnt restart, taskbar was froze, and task manager wouldnt work. Its weird.