Instructions
Download the Games for Windows Marketplace application if you have uninstalled it from your computer by visiting the download page on Microsoft's website. Install the application by following the instructions.
Launch the application by clicking on the "Games for Windows Marketplace" shortcut in your Windows Start menu or on your desktop.
Type in your Windows Live ID and password in their respective fields and click on the "Sign in" button.
Click on the "Downloads" tab. You will now see which games you have purchased through the Games for Windows Marketplace application. Click on the "Download" link next to the game you want to redownload. The program will now download the game and install it automatically.