Instructions
Open a Web browser window and navigate to the prepaid code redemption page at Xbox.com.
Type the Windows Live ID associated with your Xbox Live account into the "Windows Live ID" field.
Type your password into the "Password" field, and then click "Sign In."
Type the 25-digit code from the back of your Microsoft Points card into the field at the center of the window.
Click the green "Redeem Code" button to add the points to your Xbox Live account.