Well, I'm far from a SAMBA expert. I've beaten it into submission a few times on various rigs, but usually it's by trial and multiple errors.
Anyhow, I might check how the permissions are set on the remote (Windows) PC. The "password" it's asking you for may be that stupid "IPC resource" password screen, to which there isn't a valid answer. What version of Windows is the remote PC, and what access controls do you have set? (Share level or User level? Any Domain or AD issues?)
Also, if user level, make sure that from the Amiga side you're providing the username as the same as the account on the remote PC that has no password set.