i got GW yesterday and noticed that it randomly restarts my computer after about 20 mins. My computer is just below recommended. the only thing under is the video card.
A Heating Issue. Most systems restart when they get too hot.
A Power Issue. If your power supply is not large enough, overclocking your video card or using too much of your system at once could stretch your power supply too thin and cause the system to crash because its internal components are simply turning off from lack of power.
If you haven't messed with your system recently and up until this issue the game ran fine, then it probably isn't a power issue.