In order to hard reset a Sony Ericsson W550, 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 W550 device will be brought back to the default configuration of a brand new device.
1. Start by downloading and installing the PC suite application on your PC.
2. Start the app and plug your turned on phone to the PC (via USB cable).
3. Go to Tools > Phone Repair. Select your phone's model.
4. Continue with the installation process and allow the phone to reset'n'restart. Well done.