Instructions
Go to the Xbox "Registration, Warranty and Repair Services for Devices" page (see Resources for link).
Sign in with your Windows Live user name and password to enter your Xbox LIVE account page. If you don't have a Windows Live ID yet, click " build your gamer profile and get your Windows Live ID." You will need to create a Gamertag name, pick out a Gamertag image and enter a motto. Then enter your email address and password, and accept the user agreement.
Select "Support" at the top of your Xbox LIVE account page. Choose "Check Repair Status."
Click on "Register a New Device."
Enter your device's serial number (which can be found on a sticker on the back of the device), your first and last name, your address, your phone number and your email address into the corresponding fields. Then select "Submit." Your Xbox 360 is now registered.