Format and reinstall Nokia Phones

Press and hold the three keys: * , 'green call key' and '3'. Keep holding them and power on your phone (its sometimes tough to press the power button yourself when holding these three keys, so you can ask someone else to press the power button for you). Don't leave any of the keys until 8 seconds, its very important. Your phone will be completely reset, your OS will be freshly installed and you will loose all your contacts, messages that were in phone memory, GPRS settings and everything that was stored in the phone memory. Your memory card and SIM card contents will be intact. All your passwords and security codes will be set to default (00000 or 12345).

PRECAUTION:

BE SURE THAT YOUR BATTERY IS ABOVE 70% CHARGED WHEN YOU START DOING IT. If your battery gets drained in the middle of the process, then your phone will be unusable. So better charge your phone for 2 hours before you start doing it. The whole process may take upto 10 minutes.

Source(s):

I  have done it many many times. It works for all Symbian phones: Nokia 6600, 7610, N72, N70, 6681, 6680, etc.