Second method:

  1. First you should download PC Companion, in example from here: http://www.sonymobile.com/gb/tools/pc-companion/
  2. Install it and run.
  3. Next you will see window with 4 options.
    HardReset SONY ERICSSON W908
  4. Choose option "Support Zone New!", and click "Start".
    Factory Reset SONY ERICSSON W908
  5. Wait for the program to install the appropriate add-on.
  6. After that you will see next window with 3 options.
    Master Reset SONY ERICSSON W908
  7. Goto option "Phone / Tablet Software Update", and click "Start".
  8. Select "repair my phone / tablet".
    Permanently delete data from SONY ERICSSON W908
  9. Now follow the instructions and accept all informations. 
    Wipe data on SONY ERICSSON W908
    Format SONY ERICSSON W908
  10. Please be patient and wait until preparing computer for software update is complete.
    Restore SONY ERICSSON W908
  11. Afterwards choose your phone / tablet model.
    Remove screen password on SONY ERICSSON W908
  12. If you see this window, then connect your device into your PC.
    Remove Screen Lock on SONY ERICSSON W908
  13. Done!
See the First Method!

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

Help! This doesn't work