Instructions
Click the "Start" menu button on the computer you want to stream the media files from, then click "All Programs," followed by "Windows Media Center."
On the "Tasks" menu in Windows Media Center, click "Add Extender." Follow all onscreen instructions until you are presented with a dialog box that says "Enter Setup Key."
On your Xbox, scroll to the "My Xbox" section of the dashboard, then highlight "Windows Media Center" and press "A" on your game controller.
Follow all onscreen instructions to receive an eight-digit setup key, then write this number down.
Enter the eight-digit setup key into the "Enter Setup Key" dialog box on your computer, then follow all onscreen instructions.
Click the "Finish" button on your computer when it is presented. This will cause the Windows Media Center user interface to appear on the TV that your Xbox 360 is hooked up to. You can now browse all media files on your computer that are compatible with Windows Media Center.