1. Depress the two left hand side buttons (camera and voice recorder) and keep them held in.
2. Use the stylus to depress the reset button on the bottom of the unit and release, whilst still holding the two side buttons.
3. Continue to hold the side buttons for a count of four or five.
4. A screen will appear asking you to press Send to confirm the re-format. Press the Send button (the green phone) and your phone will be hard reset.
To avoid the network specific software being loaded run through the "copy and past" exercise, set the time zone and a password if you want one. The phone will then tell you that it's going to customise itself (or somesuch message) in three seconds. Use the stylus to reset before the end of the countdown and you will avoid the installation of the network specific software.
WARNING: This means that you may not be able to connect to GPRS, send or receive MMS and may lose some functionality supplied by your phone network. If in doubt (and if you are unable to install from the EXTENDED_ROM cab files) then it may well be best to let the networks specific software install itself.