1. At the start you will need offical Sony Software called PC suite to succeed this operation.
  2. Download this software from the offical -> Sony Site  
  3. Now power up your device by pressing Power buttonHard Reset SONY ERICSSON P800
  4. Install PC suite software on your PC,then start appliaction and plug in your device.
  5. When application will start go to Tools then select Phone repair, find and select your device model. 
  6. Follow up with instalation wizard.
  7. When PC suite will end Factory data reset your handset should restart.
  8. Success! phone is ready to work on.

Hard Reset will erase all of your data
All described operations you are doing at your own risk.

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