The only way is to reformat it to put it back to it's original state when you just bought it. Here is the detailed procedure on how to do it,
mustumasti gave this a few months back. And I must say that this HELPED a lot of people I know.
Hope it could help you too.
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Subject: How to reformat a Nokia 6600 Here it is, to let everybody know how to perform a full phone formatting on a Nokia 6600.
If you encounter situation like below on your 6600,
1. Blank screen phone unable to reboot.
2. Phone only able to boot-up to "Nokia" word screen.
3. Install some program but not able to uninstall it after that.
4. Unable to delete unwanted files on C drive.
Do a Full phone formatting on your 6600 and follow steps below.
1. Make sure you have at least 3/4 charge of battery power left.
2. Backup your contacts list and personal files to MMC memory card.
3. Switch-off your 6600 phone.
4.
Press and hold 3 keys;
Green dial key,
* Star key,
no. 3 key on keypad and then press the power on/off to switch on the 6600 phone.
Remember, do not let go the 3 press and hold keys until you see a formatting word screen show!
5. After a few minutes when the Full phone formatting completed, your 6600 will back to original system and factory setting.