And the file-size of that file or those file are wrong.
And will therefore give you an error like you said now.
Follow this and your problem should be fixed: (PS: If you made backup just do that instead of the video.)
By doing this you will retrieve the original game files.
Since I suppose you got steam version of the game just using Alter MW3.
And then you just do what you need to do to be able to use Alter MW3 again.
(So you have to re-install Alter MW3 but not MW3 itself.)
I hope you got some answers.