Close down MW2, restart it. Don't click "Play" yet, because you can only go back to fullscreen in the FIRST menu you see.
Simply do the Alt + Enter again and it's back fullscreen. Edit your resolution to your preference after.
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where in the hell do i download this fucking noobs you only say step 1 step2 step 3 but not somthing els!!!!!!!!!!!!☆┌─┐ ─┐☆