That is probably your problem. This post should help you with that:
http://node2.projectplanet.cn/@forums.steampowered.com/forums/showpost.php?p=22330740&postcount=2098
I can confirm the validity of this solution. Best of luck to those of you still struggling!

http://node2.projectplanet.cn/@forums.steampowered.com/forums/showpost.php?p=22133750&postcount=1032

so our prayers are finaly answerd? did valve realy pull it off? but we still hve to wait for them to release the patch. keep your fingers crossed people ^^

...What are you still doing here? This group is mainly for people who can't even reach the menu/start a Single Player game.

At 164 Hours, Portal has become our most played game of the group. ...You see how desperate we are, Valve?

Test subjects, whether you've been selected by Aperture or just had the (mis)fortune of participating in this experiment, your current task is one of many choices and two ultimate outcomes. Depending on your actions during this test, you may either A, be able to play Portal 2, or B, have to wait until Valve releases a patch that makes the game playable on your computer. For some of you, the desired outcome may be more achievable than it is to others. "Diversity" is a trademark of Aperture Science, and it is understandable that there will be a variation in individual experiences during the course of the test.
With that said, we are now ready to begin the test proper. Let the testing... begin.