Oh, people are still posting in this thread.

In my case as strange as it sounds, it was gfx card causing these reboots during startup of Windows 7 in the login phase. It turned out the EVGA GTX 260 SC was getting more and more unstable at the clocks and in the end it wouldn't even work at stock clocks and got worse on a daily basis. I wouldn't have thought gfx card would be able to do something like this if it still ran games fine at the time but could cause reboots during startup.