Hi Karel,
Just wondering if that device appears because you have not installed the Microsoft ActiveSync yet in that computer.
When the BenQ T80 enters in firmware-can-be-written mode, it is detected as a device called "Neptune". In that moment, the file BenQE21Ser.inf is the descriptor for installing the correct driver. After installation, you are able to use the application to update its firmware.
Cheers!
Jose