When you have a user in 'admin users', samba will always do file operations for this user as 'root', even if 'force user' has been set. Newer windows clients(NT4, 2000, XP) send encrypted passwords. Samba can't compare these passwords to the unix password database, so it needs it's own user database. You can add users to this database using "smbpasswd -a user-name". See also the "User database" chapter of the samba HOWTO Collection.