Modern Warfare 2 "out of range " issue

I was happily playing MW2 until my 5 year old changed the screen resolution of the game to one my computer cant display , so now when  MW2 multi-player starts it gives a black screen , "out of range " message . How can I get back in to change the resolution ?? game works fine in single player mode but not in multi player mode

I have tried re-installing game but no good .

I had the same issue with Dawn of War 2 but added "window" in the set up launch option in steam and got into that game that way to change resolution .

    how can i do it with COD 4 MW 2
  ????