In order to hard reset a Sony Ericsson P800, 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 P800 device will be brought back to the default configuration of a brand new device.
1. Download the Sony application PC Suite.
2. Turn on the phone.
3. Now install the PC Suite on your PC, start it and plug your cellphone to your computer.
4. Open Tools and select Phone repair. Select your phone's model and continue with the instalation wizard.
5. After the instalation is complete, your phone should reset'n'restart. Voila, done.