If you bought it from a reputable dealer, and they are local, then they will do updates for you free (apart from the Garmin/Jepp $35 charge).
Until the 296 serial was the normal comms, and all PCs at te time had RS232 plugs, but technology moves on. With a good USB/serial convertor (Belkin usually work, some others are iffy) then the update software will fin the unit automatically. When you say serial to USB cable do you mean a convertor?