dear catymag,
Thank you for reading and answering.

I (re)installed usb drivers, and followed the link, I do have a original cd and product key but the Garmin is still not recognized as a Garmin, only as a drive letter.
When I " safely remove device " it shows up as Garmin usb!, but otherwise windows suggest to format.
I tried to install some software I found : nuvi_sys.rgn and nuvi300_540. At that time the display showed up with the message : "loader". (I was happy for a short while, but no cigar)
From the secret pages inside the Garmin (accessed by holding down the upper right corner from the display) I learned that I lost the ROM and the Basemap.
Both failed to pass during test. RAM and ASIC passed.

Should I format the Garmin at this point or should I continue by trying this way?
Have a nice day!