Can't launch Portal 2

["focait"]

I can't seem to get portal 2 to launch.
Whenever I try to launch it, I get a window saying "Completing installation x%" with X being the percent completed, and once it gets to 100%, it simply gives me this error message.
https://www.dropbox.com/s/pds8k2t3svai7yx/failure.PNG
I've tried reinstalling Portal 2, and restarting steam, but I still can't get Portal 2 to work.
Any ideas on how to fix it?