Use the three button combination. Switch off the phone. then press the three buttons (green button, * , 3) and switch on the phone. the phone will switch on and display formatting on the screen. you van then let go of the buttons and wait for the format to finish. Make sure your battery is fully charged or the charger is connected.
RESETTING PHONE BY NOKIA SECRET CODES (steps 1 and 2)

1st step : 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
*#7780#
It will ask for �Restore all phone settings-phone�ll restart�.�
Then it asks for security code enter ur security code whose default value is 12345 then the phone is resetted after restart.The phone will be restored to the time when you purchased the phone.Default apps are not deleted.

2nd step : 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 apps at d time of purchase are deleted.The phone�s setting will be restored to the time when you purchased the phone.Default apps are not deleted.

Just enter the code-
*#7370#
Then it asks for security code enter ur security code whose default value is 12345 �.then proceed as in previous step�..

NOTE:
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 .