"Have you tried turning it off and back on again?" j/k
Z, I often keep a copy of
Ultimate Boot CD handy. I find memtest86+ works well. You need to let it run over and over, leave it go while you sleep. (Chances are the errors that cause windows to reboot the computer won't cause this to)
You also might want to check and see when you updated Windows last and what third party software and/or updates you installed last and restoring previous to that. Or likewise could try loading the latest updates.
You've also checked (in a hurry so not rereading everything, lol) any exhaust openings and fan openings for proper clearance on the computer correct? I don't know if you have air conditioning or anything, and it has been getting warmer recently. (What time of the year did the previous system fail?)
-MJ