Windows reboots itself only on startup.

Hi,
Well, when I first start the computer the next day it restarts itself a few minutes after loading windows, then I go into safe mode and restart back to normal mode and the computer runs fine afterwards. It happens almost every time(on rare occasions it works fine) when I first start up the computer. It use to lock up instead of rebooting but that was solved when I uninstalled AVG anti-virus scan, so I have no idea whats happening... please help.. Thank You..