Instructions
Visit your local game store or electronics retailer and buy a physical copy of "Modern Warfare 2." You should be able to find it in the computer games section.
Insert the first install disc and click "Install" on the window that pops up on your screen.
Accept the license agreement that is presented when you first install the game. This agreement details how you may use the product.
Enter your unique product identification code when prompted. This code is usually found on a sticker on the back of your game manual.
Select "Modern Warfare 2" on the game select window and hit the "Next" button.
Insert the second install disc when the installer asks for it. Once complete, the game will be ready to run.