I don't think that answer is correct
I don't think changing the force group or user is the best answer.
I think I am in the same boat as the original poster: I have two users in the smbpasswd file: "nobody" and "me". When I connect to the share, the User name is greyed out and is promoting me for the password for server\Guest.
Even if I do set the force user to "me", "server\Guest" is still in the greyed-out User Name field. Even if setting "me" to the forced user worked, it is supposed to force anyone's identity to "me" if they were to succesfully log in. I don't think that's what the OP intended. although it might be a workaround.
My guess (I am not a Windows authentication expert) is that this is something the XP client and not samba is forcing upon me, and I need better rights to make the client login actually prompt me for a username, at least without tying my samba into the local AD domain.
# Global parameters
path = /app/me/Docs
valid users = me
force user = me