If it is happening in either OS, I'd say this sounds definitely like some sort of hardware issue.
First, lets do s quick check of the capacitors and power supply. You can test by opening the case up and using a flashlight to check the capacitors on the motherboard. They look like this: http://www.thenakedpc.com/dan/Bulging_Capacitors/close-up.jpg
Good capacitors have a flat top. Bad ones will be curved or leaking on top. A slight curve still means it is bad. Make sure they're all nice and flat before you move on.
If have a power supply tester, or can borrow one, test the power supply to just make sure the voltages are correct and steady.
Second, I'd suggest re-testing the memory using Memtest 86+ (http://www.memtest86.com/
). Also try the Windows Memory Diagnostic tool (http://oca.microsoft.com/en/windiag.asp
) -- sometimes memory will fail one, but not the other. Run them each for 2 hours, just to be sure.
Once you're finished with these checks/tests, post any results you find so we can narrow things down.
Edited by Suicide King, 24 March 2011 - 10:25 PM.