Instructions
Press "A" to sign into your gamer profile from main Xbox 360 menu. This will connect the system to Xbox Live.
Scroll up and select "Xbox Live Marketplace" on the Xbox 360's main menu.
Scroll up and choose the "Games on Demand" list of available downloadable games. This will display a list of all of the original Xbox games that can be downloaded to the Xbox 360.
Highlight a game you want to download and press the "A" button. A menu will pop up providing more information about the game, including the game's price.
Use the "A" button to select the "Purchase" option. Downloading an original Xbox game can take several minutes, depending upon the size of the specific game and the speed of your broadband Internet connection. Once the download is finished, the game can be played immediately from the "My Xbox" portion of the Xbox 360's main menu.