Instructions
Open your disc tray and insert your retail copy of "Modern Warfare 2." Close the tray.
Right-click the Steam icon in your taskbar. Select "Exit" to fully shut down Steam.
Press the "Start" and "R" keys on your keyboard simultaneously. This brings up a Run window.
Type the following into the text field, with the quotes, replacing "X" with where you chose to install Steam and replacing "Y" with your CD/DVD drive letter: "X\Steam.exe" -install Y:
Press "Enter." Steam will restart and ask for your login information. Type it in, and the installation will continue directly from the disc.