Go to control panel/system/Advanced tab/Start Up and Recovery. See if Automatically reboot is checked and if so, uncheck it. This is somebodys idea af a useful item that if something goes haywire, the machine automatically restarts without giving you a choice. I had the same problem as you have and this cured it. After wards, i began to see entried in the system log about what was causing the problems, and i got the guidance here to cure those problems