Instructions
Turn on the console and wait for the main menu or "Dashboard" to load. Hold the center "Xbox Guide" button until a green LED appears on the controller to turn it on. Move the analog stick to the right and select "System Settings," then press the "A" button. Select "Console Settings" and press "A" again. Select "System Info" and press the "A" button. The system will display your Xbox 360 serial number which identifies the machine.
Unplug all cables and USB devices from the machine if you wish to locate the console's serial number without powering it on. Look on the rear of the machine next to the A/V cable port. The serial code for your Xbox 360 will be printed above the A/V port.
Unplug all USB devices from the front of the machine. Push the small door on the USB ports until it moves and a sticker is visible below it. Your 12-digit identifying Xbox 360 serial number will be printed on this sticker.