Instructions
Click the "Start" button on the Taskbar and select "Computer."
Double-click the hard drive where Steam, the application required to launch "Modern Warfare 2," is stored. For many users, this is the "C:\" drive.
Double-click the "Program Files (x86)" folder followed by "Steam."
Double-click the "Steamapps" folder, followed by "Common," "Call of Duty: Modern Warfare 2" and "Players."
Right-click either "Config" or "Config_mp," depending on whether you want to edit your single player or multiplayer resolution. Select "Open With..." followed by "Notepad" in the pop-up window.
Navigate to the "Seta R_Mode" entry in the file. Change the resolution in the quotation marks to your desired resolution. For example, setting "1440x900" sets the game in that resolution.
Select "File" and "Save" to save all changes.
Launch "Modern Warfare 2." The game launches at the resolution specified in the configuration file.