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#21
Run the maker's extended HD diagnostics CD scan, test the RAM, triple check all cable connections, change AV to MSE and work through the other steps to go over your install in the Troubleshooting Steps for Windows 7.
You seemed skeptical about Windows Update so it makes me question your install. Did you enable Automatically deliver drivers via Windows Update (Step 3) then run all rounds of Important and Optional Updates, with reboots, until no more were offered - before you imported any drivers that Win7 might not want? This is a driver-complete OS via the installer and Updates, with mistakes one in a million.
So if you used XPired methods I would consider reinstalling but this time follow these same steps as for a Clean Reinstall - Factory OEM Windows 7 which compiles everything that's worked best in tens of thousands of installs we've helped with here. This time install nothing (other than browser and desktop setup) besides the Updates checking performance after each round. Then test performance longer before starting program install and test performance after each one.