I found this very useful article on formatting/flashing a nokia phone..
SOFT RESETTING NOKIA 6600
A Soft-reset is the process of resetting all the settings of the phone to the factory default! No applications are deleted.
Take backups of ur phone books,calender and others, then simply enter the code
It will ask for “Restore all phone settings-phone’ll restart….”
Then it asks for security code enter your security code whose default value is 12345 then the phone is reset after restart.The phone will be restored to the time when you purchased the phone.Default applications are not deleted.
HARD RESETTING BY CODE
A Hard-reset is like formatting a memory disk. It formats the phone memory. Everything that has been installed other than default applications at the time of purchase are deleted.The phone’s setting will be restored to the time when you purchased the phone.Default applications are not deleted.
Just enter the code-
Then it asks for security code enter ur security code whose deafault value is 12345 ….then proceed as in previous step…..
The battery must be full or the charger has to be connected to the phone so that it does not run out of power and make the phone unusable.
Take backup of the contacts and other important data in MMC .
’FLASHING 6600′ : ANOTHER METHOD FOR HARD FULL FORMATTING
If any of the above methods failed to make your phone formatted then try this step-mainly try this step if the problems are following-blank screen phone unable to boot, phone is only able to boot-up to Nokia screen ,unable to uninstall any installed software, unable to delete certain files on C drive(phone memory).This step is known as flashing of phone. Always do this step at last (after 1st 2 steps are over and u are not successful) as these problems might be solved by resetting using codes.
1. Make sure you phone is fully charged. (At minimum 3/4 of battery should be left)
2. Backup your contacts list and personal files to MMC memory card (if possible).
3. Switch-off your phone.
4. Press and hold these three keys together
-Green dial key,-Star key (*)-Three key (3)
then press the power on/off to switch on the phone.
Do not let go the three keys until you see the “formatting” screen displayed
5. After a few minutes, the format will be complete. Your phone will now be at its original factory settings (It will reach the same state as that in when you purchased it). All inbuilt applications provided by nokia will not be deleted)
Disclaimer: We will not be held responsible for anything that happens to your phone as a result of following this guide. Use this guide at your own discretion and risk.
Source: orkut.com & http://nokia6600.wordpress.com/2006/05/18/nokia-6600-formattong-how-to-format/