You've got a hardware problem. It might seem to be related to your AV software but that's likely only because it is the first kind of software you install that really taxes the system. I imagine you'll get the same result if installing and running a game with sufficiently high system requirements.

Quote Originally Posted by clh333 View Post
One pass through memtest86, no errors:


One pass is not enough. Let it run overnight. I once reduced the CAS latency on a system of mine and memtest86 found no errors (even after several passes, IIRC). Happy as a clam over my apparently successful little "overclock", I proceeded to boot Windows and run Battlefield 2. After playing for a while (half an hour maybe?) I was disconnected from the server because I had failed the integrity check internal to BF2. In other words, the RAM contents was corrupt.

The CAS latency went back up and I was sad that BF2 was back to its regular, somewhat laggy, self. At the same time I was grateful that I had not caused any file system corruption. Anyway, the point is that you might need to run memtest86 for a long time before errors are found. The same goes for Prime95, which you still haven't run, it seems. Reinstalling Windows and AV software over and over again isn't going to change anything.

Also, if you haven't already done so, I would recommend doing a visual inspection of the motherboard to look for bulging or leaking capacitors. After all, this board is straight out from the bad caps era.