NT user authentication means that when client machines use HTTP proxy or SOCKS services, the user will be prompted for a name and password, and
that name and password will be vaildated against the NT users and passwords available on the NT server.  This also works for Windows 2000 and Windows
XP servers.

Where the users and passwords come from depends on how you have your server and domain configured.  If the users and passwords available on your
server come from a domain controller, then that's what will be used.  If it's a stand-alone server, then only the names and passwords defined on that
server will be valid.