Instructions
Power on the Xbox 360 and then go to the "My Xbox" section. Select "Windows Media Center." Verify your gamertag and network settings. A network test will be performed, and once it is completed you can press "Continue." An eight-digit code will appear.
Power on your Windows Media Center PC. A bubble should pop up that notifies you that an Xbox 360 device has been found. Click the bubble and a new window will pop up. If you do not see the bubble, open Windows Media Center and click on "Tasks" and select "Add Extender."
Enter the eight-digit code when prompted. Click on the "OK" button to complete the link. You will now be able to access your PC content on your Xbox Live account.