Computer crashing after playing a few games and then start

I'm new here don't know if this is the right forum to post in. I recently installed Windows 8 64 bit from Windows 7 32 bit. I never had any problems really with the windows 7 32 bit but since upgrading to Windows 8 64 bit I am having this odd problem. I am a gamer, I have Steam with lots of games and so I play one then play another and another then I go to start a game (It isn't the same game)...