If you encounter a situation like below on your symbian phone,

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. Error messages such as 'Child Installer' keeps popping-up.

do a full phone formatting on your phone as 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. (see link below)
3. Switch-off your phone.
4. Press and hold these 3 keys; Green dial key, * (star key) and no. 3 key and then press the power on/off to switch on the phone.

Remember, do not let go all of the keys, hold until you see a formatting word screen show!

5. After a few minutes when the full phone formatting completed, your phone will be back to original system and factory setttings.

All applications installed will be lost unless you've made a backup on your mmc. You can restore the backup to the C drive again.