Do you get any Blue Screen Error or any kind of error message or Beep Codes?
Go to "System Properties" (press Windows Logo button if you have it on your keyboard + "Pause/Break" button. Or do a Right click on My Computer) Click the "Advanced" tab; click on "Settings" under "Startup and Recovery" section. Make sure that, under the "System Failure" section, the "Automatically Restart" option is UNCHECKED. Reboot and see if you get any error or Blue Screen, or post back with more info. Good Luck!