To authenticate users on Windows machines, the windows machine must join the domain created by samba server (i.e. BLUEPRINT-NT). To join this domain, login to the computer using the administrator account. Then right click my computer icon and select Properties-> Computer Name -> Change. In the new window, specify the computer name and the domain name (BLUEPRINT-NT) and click OK. A new window would pop out asking for the username and password of the domain controller. Enter the root user and its password (or any other samba users), and the computer will join the new domain. Restart the computer to let the change take effect.