Knowing more about Windows network administration than i do about PHP, i have to wonder what exactly you are trying to accomplish in this situation.
if all users are going to be able to access all pages once they are logged in, and assuming you are on a domain network, then it seems like considering the username is beside the point; which is to say that once someone has authenticated on the domain, that should be enough to allow them access to the intranet.
However if you're going to restrict some user's access to certain pages, than i don't believe you're going to be able to get around implementing user authentication in the web server database (mysql, postgre, etc.) The only solution i could even fathom would be for the webserver database to utilize the same authentication database that the domain uses. This sounds like something that might be able to be done with IIS, but these are just wild theories to me (a humble apache user).
And, in the end, it seems like if they can remember their user/pass to log on to the domain, why cant they just use the same login information when they register on your webserver database?