In order to hard reset a Sony Ericsson K790, 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 K790 device will be brought back to the default configuration of a brand new device.

1. Switch on the phone.

2. Go to Menu > Settings > General > Master Reset.
3. Press Left Softkey to confirm the process. Follow the on-screen instructions.

4. Phone will now reboot and restart. Done!