Normal Reset

Dial *#7780#. 

This will restore ini files from ROM, keeping all your data (themes, third party applications, 
video, photos, and music files). 
Deep Reset

Dial *#7370#. 

This will completely reformat the phone memory. Default files are cleaned and rewritten. 

Note that data found on the memory card will not be altered.

Source: http://en.kioskea.net/faq/421-restoring-your-nokia-phone