Creed FFXIanier
The config file is where your Modern Warfare 3 is installed, in the players2 folder (plutonium.cfg)
Try to put your game in windowed borderless with this:
Find r_displayMode and change it to be like this (you can use ctrl+f to search for it):
seta r_displayMode "windowed (no border)"
What Hannes said above is for Black Ops II not Modern Warfare 3