How to reset/reformat a phone:

It may happens that a program corrupts the C: drive of your phone. In such case, some data can be lost of course but, more annoying, some applications may not work or work only partially,…
Series 60v3

Normal Reset (*#7780#) : Restores ini files from rom but preserves user data (photos, 3rd party apps etc)

- Deep Reset (*#7370#) : This reformats completely the C: drive. All applications and files stored on this drive will be lost and clean default files will be rewritten.

In both case, the phone will ask you a confirmation and you will have to enter a security code (12345 by default).

Files and applications stored on E: drive are not be affected by these sequences.