Thanks Ken
I found a solution by chance, by removing the sd card which was bugging me each time i plugged in the reader on windows.
It seems that the presence of a SD card was kind of bugging the kobo.
I removed the sd card, but i also installed the kobo desktop on a Win7 computer, showing me that the device has an account
xxxx@xxxx.xx, so i unplugged the kobo and it was working.
So i blame the sd card for all the problems and i won't look further.