Google a program called Autoruns and download it and disable all the stuff that is not from microsoft and the bho's under Internet Explorer that are NOT microsoft, reboot and test.
Also, check your cpu usage, something may be misbehaving and slowly wasting memory, until you run out two hours later. That would likely cause the os to just fail and go into a reboot. Microsoft has made a lot of efforts to remove the blue screen of death... so now you get reboots instead... when the system becomes unstable.