Need help with my alienware 51x Desktop issue

hey Alienware, I have problem with my x51 that i bought like 2 weeks ago, and i used it today for the first time, and when i switch it on it runs good, and when i use it to play games or watching youtube it crashs after 10/20 mins and shows alien head picture with no icons or mouse cursor and it stucks and i cant do anything.. Please help me :((