if you're RAM isn't failing, which it probably isn't because the machine isn't failing during POST, I have a good feeling it's your page file that is corrupt. You can delete it and the machine will recreate a new one when you restart. PM me if you would like detailed instructions. (It could be a result of your frequent defrags).

If that is not the problem, there could be a bad spot on your disk, which is more difficult to rectify without reformatting/reinstalling and/or putting a new HD in the box.

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