In order to hard reset a Sony Ericsson Z800, 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 Z800 device will be brought back to the default configuration of a brand new device.
1. Turn on the phone.
2. Select Menu > Settings > General > Master Reset.
3. Continue by confirming and following the on-screen instructions.
4. Voila, the phone should now reset and restart.