Instructions
Launch the Steam software on your computer by double-clicking on the Steam icon on your desktop or under "All Programs" in the Start menu.
Click on the "Library" tab in Steam. If prompted enter your Steam user name and password. If file updates and patches start to download (as indicated in the status bar at the bottom of the Steam browser window), wait for the download completes before proceeding.
Find "Call of Duty: Modern Warfare 2" in your Steam library in the "Game" column under "Library." You can scan manually or type "Modern Warfare 2" in the "Search" box at the top of the "Games" column.
Right-click on the "Call of Duty: Modern Warfare 2" listing and select "Delete Local Content." A prompt will appear confirming this choice. Choose "Yes" to continue.
Wait for the "Deletion" progress bar to fully complete and restart Steam and the computer if prompted.