No, don't trash anything, believe it or not you're having better luck then most, at least you can see it. First try this when prompted, if you see an option to choose other user or something like that choose that if not or in either case try using this format machinename\username and then the password, that is
TheVistaDestopMachineName\UsernameThatWorksOnTheVistaDesktop and then the password for that user. Alternately you can try adding the username and password you are using on the Win7 machine to the user accounts on the Vista Desktop. There seems to be some issues with missing passwords on some home networks, if you are not using passwords you may have to take that leap or perhaps turn of password protected sharing