Turn on the Xbox 360
Press the Guide button on the controller
Go to Settings
Select System Settings
Select Console Settings
Select System Info
Note the console serial number if needed
Go back to Settings
Select System
Select Storage
Highlight the hard drive or storage device
Press Y
Select Format
Confirm the format
Go back to System Settings
Select Console Settings
Select System Info
Press the following button sequence on the controller:
Left bumper
Right bumper
X
Left bumper
Right bumper
X
Confirm the factory reset if prompted
Restart the console
Set up the Xbox 360 again after reset