Hmm, I had the same problem with MW2 I'm not sure if this is the fix or not as it was a while back but try logging out of your steam account and logging back in.
Logged out and logged back in, at the minute no error comes up but it doesn't open, and it says it is. Looked in task manager, had to go to processes to close it.
EDIT:
logged out again and the stupid errors back :S.