Things You'll Need
Instructions
Navigate to the main Xbox homepage (see link under Resources).
Click on the "Support" link on the right side of the page.
Under "Console Management" in the middle of the left side of the next page, click "Register a Console." A new window will open.
Click on the green button labeled "Register" in the "Register a Console" section on the bottom left corner of the page.
Sign in with your Windows Live ID, and follow the instructions. It will ask for some personal information, as well as your console serial number. There's a diagram on the page to help you find your serial number.