When you say you reinstalled, did you run the OS restore from the image on your drive, or did you perform a fresh installation?
One thing you'll want to do to diagnose the problem is to turn automatic restart off, as described here:
http://pcsupport.about.com/od/windowsvista/ht/arestartvista.htm
I have had my Dell Inspiron 531s slimline for about a year with no problems. I have only added an optical mouse and a printer to the computer. The computer came loaded with Vista and I have never had to reload my OS. For some reason it has begun to shut down and restart randomly for no reason. I reinstalled Vista and the problem persists. I tried the heatsink and it seems to be seated fine. I am at a loss as to what it could be as it could be a heatsink, processor, fan or a number of other things.

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