You can now do the following online:
You can register your Xbox console to associate your console with your Windows Live ID and to save your contact information in our registration database. If you don't have a Windows LIVE ID, you can sign up for one here.
For further details and a step by step walkthrough you can go here: Xbox 360: How to use the Xbox Service Web site.
Please visit the Xbox Service Web site to set up your repair, to check your warranty status, or to check the status of an existing repair.