Could not load localization .txt

I receive the following error when trying to start MW2 

"Could not load localization .txt. 
Please make sure Modern Warfare 2 is run from the correct folder."

I've seen people with this error before, but have yet to see any solutions.  I have verified the game files multiple times, to no avail.  I am in the U.S., and have been playing the game since release.  This problem just started today.  Any thoughts??

Thanks!