Win7 doesn't even take a gig to run, I have it on my AMD 3400 x2 with 1054MB ram and it is fine. Did you install Win7 yourself or did it come with it? I woudl start with a fresh install of Win7, if it still happens then turn to hardware. If you're not using the standalone ATI video card, remove it and see if the problem persists. Essentially you want to keep narrowing down the problem until you just have HD, Motherboard, and Power supply. Then you know it is one of those three or RAM.

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TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth