In order to hard reset a Sony Ericsson C903, you should perform the following steps.
Remember! Hard reset will delete all the data on your device; pictures, contacts, messages, apps and more.
Sony Ericsson C903 device will be brought back to the default configuration of a brand new device.
1. Start by installing the official Sony software called PC Suite. I suggest getting it from Sony official website.
2. Switch on your phone.
3. Install PC Suite on your computer, start it and plug your phone to the PC.
4. Go to Tools > Phone repair. Find and select your phone model.
5. Continue with the installation. Your phone will eventually reset & restart. Unplug your phone. Done! The hard reset is now complete.