Instructions
Before attempting to send your console to Microsoft for repair, use the troubleshooting guides provided on Microsoft's Xbox support page to ensure that your console is truly broken (see the link in the References section).
Select the ̶0;Register a Console̶1; link once you are certain that your console requires repairs. You can find this link at the bottom of the navigation menu on the left side of the page--under the ̶0;Console Management Tab.̶1; You will need to supply your name, address, contact information and your console's serial number.
Select ̶0;Request a Repair̶1; once you have finished registering your console. This link will appear next to an entry describing your console in the middle of the page. Alternatively, you can follow the ̶0;Repair Your Console̶1; link, which is located directly underneath the ̶0;Register a Console̶1; in the ̶0;Console Management̶1; tab.
Specify the problem you are experiencing and decide which shipping method to use. You may either print a shipping label and package your console for return yourself, or you may request that Microsoft send you the shipping label, box and packing materials. If you decide to request a box from Microsoft, it should arrive in three to four business days. You will also be apprised of your warranty status. If your warranty is still valid, repairs are free. If your warranty has expired, repairs will cost you $99.99 (as of 2010). However, if you are experiencing the three flashing red rings or the E74 error, your warranty is extended to three years from the date of purchase.
Remove your Xbox 360's hard drive and check your disc tray to ensure that there isn't a game inside.
Package your console, attach the shipping label, and send it to Microsoft.
Track your console's status by using the ̶0;Check Repair Status̶1; tab. You will probably have your console, or more likely a refurbished console, back within a week or so.