You should be able to do so by entering "*#7370#" - this will do a "hard reset" which restores the phone to factory condition ("out of the box"). I believe the phone will request your security code to complete the reset - it should be "12345" if you've never changed it from the factory default. All user installed content will be lost; so if it's important to save it, be sure you've done a backup with Nokia's PC Suite first.
