Buy Killer is Dead

Use your gamecard Microsoft Points in the Xbox Live Marketplace of your Xbox 360 to purchase full games, rent the latest movies in High Definition, add downloadable content to your games. Exploit the full potential of Xbox Live!
To access the Xbox Live Marketplace, you need the following components and services: Xbox 360 Console, broadband internet connection, Xbox Live Silver membership (included in the product contents of the Xbox 360 entertainment system), Microsoft Passport account, Hard Disk or Memory Unit for the Xbox 360 Console.

For Steam CD Keys, you must download (HERE), install the Steam software and create a new account. If you already have a Steam account, once you log in, click "Add a game" (located in the bottom left corner), and then "Activate a Product on Steam". After a few more clicks, you will be prompted to type in your game code. Thus, the game will be activated and automatically added to your account library.