For the SMB connection from the Win system to the Linux system, you are getting a user/pass probably due to the default /homes share in Samba, and because Guest is enabled. Check your smb.conf and make sure you have your shares setup, and set your users with smbpasswd (don't forget to enable them with -e after creation).
As for the Win system. Do you have a share on that system? If so, why not go to it directly, instead of trying to browse the system? You might also want to try the IP address of the system for troubleshooting purposes (NetBIOS really shouldn't be trusted ).