First of all, you should check to make sure your computer is powerful enough to handle GRAW. It's a very new game and requires top of the line hardware. From the specs in your sig, it looks like you should be able to handle it, at least on low settings.
Just to make sure, check out this site: http://www.srtest.com/referrer/srtest
and allow it to install it's software and run a test on GRAW. If any of the areas fall short, this could be your problem.