Why does my computer shut down without warning

Tech Support Guy System Info Utility version 1.0.0.2
OS Version: Microsoft Windows 7 Professional, Service Pack 1, 64 bit
Processor: AMD FX(tm)-4100 Quad-Core Processor, AMD64 Family 21 Model 1 Stepping 2
Processor Count: 4
RAM: 16365 Mb
Graphics Card: NVIDIA GeForce GT 610, -2048 Mb
Hard Drives: C: Total - 466650 MB, Free - 360577 MB; D: Total - 953859 MB, Free - 494770 MB; E: Total - 10239 MB, Free - 1856 MB; H: Total - 476937 MB, Free - 432067 MB; I: Total - 1907726 MB, Free - 432505 MB;
Motherboard: Gigabyte Technology Co., Ltd., GA-970A-D3
Antivirus: McAfee Anti-Virus and Anti-Spyware, Updated and Enabled


I hope I am posting this in the right place I am not sure if my problem is hardware or software related.
just rscently my computer started to shut itself down without warning. The monitor will just go black all sound stops I then have to turn it back on again using the power button.
IT usually happens when I am playing one of the games that I download from 'BigFishGames.com' It has also happened when I was watching a netflix movie. Can anyone tell me what might be causing this to happen and and if so what I can do to eleminate the problem.
Thank you for your help.
Frnknitty