Instructions
Press the "Open tray" button on your computer's DVD drive and insert the Fallout 3 DVD into it. The Fallout 3 has an autoplay feature that will automatically launch the game's installer.
Click on "Install." Then click on "Next."
Type your Fallout 3 license key in the field. You can find your license key on a sticker inside your game's packaging. Then click on "Next."
Select "Express install" and click on "Next." The installer will now decompress and install the game's files, which should take less than five minutes to complete. When it has finished installing the game onto your hard drive, click on "Finish."