Go to C:/Program Files/Steam/Steam Apps/common/call of duty modern warfare 2/players.....
And open the "config_mp.cfg" in NOTEPAD
Once its open in Notepad go to Edit->Find and type "fullscreen"
Immediately after the word fullscreen you will see a number in quotes like this : "1"
Change this number to a "0" and SAVE the file.
Start muliplayer and you will see that it starts in a window.... Go to your graphics options and under advanced make sure the "Screen Refresh Rate" is set to 60 (or what ever your monitor supports, mine was set to 56Hz which failed horribly so I reset to 60.)
Hope this helps.