Computer keep rebooting

Hello!

I have PC with Q9650, 4GB memory, PNY nVidia 1.8GB videocard, Windows 7 Ultimate, ESET NOD32 5.0.95.0.

Everything was fine, but a few weeks ago I got system restart and it start restarting from 2-3 minutes to 15-30 each time and if I boot normally.
I cleaned fan on CPU, and check a temperature with RealTemp and HWiNFO32 applets, but everytnings fine with cooling.
Except CPU temperature keep about 67 - 78 degrees of Celsius. After few cleaning it still restarting, but give me more times between restarts,
sometime a from 5 min to a few hours after a few hours of a break, and CPU temperature around 44-50. I replaced CPU with new one, tested PSU with DMM,
and tested memory thoroughly.

I can't be sure if it software or hardware issues, and if this forum is a proper place, otherwise I'll move it to the hardware forum.
But it seems like more a software problem, because I tested all hardware parts.

I don't want to reinstall Windows 7 and rather will try fix it (of course if it a possible to find a solution).

There is my Speccy report:

http://speccy.piriform.com/results/pD24EFEwg3gLDf0vrUJcvIS


Thank in advance and please help!
(English is my second language)