On the screen where it lists the startup options (Safe Mode, Last known good, etc.) there should be an option labeled "Disable Automatic Restart on System Failure" Choose that one and reboot again.
After reboot, if you get back to the same screen, choose Normal Mode.
Now, if and when Windows encounters a problem, instead of automatically restarting, it will present you with a blue screen with debugging information on it. Copy down the information on the screen (refer to the image below for an example.)
Once you have that info, power off and post it here. This info will help us figure out what's causing the problem.