Yes! 

You need to install the AD provider from http://dnnauthad.codeplex.com.  There are instructions on how to setup the provider here:  http://local.dotnetnuke.com/Developme....

I'm not sure of your level of familiarity with DNN, so I'll assume low or none and that your intranet will be at "http://intranet/".  After installing DNN and setting up the Auth provider, you'll only allow page views for the "Registered Users" group.

If you're in an active directory environment, set your users homepage to something like http://intranet/DesktopModules/AuthenticationServices/ActiveDirectory/WindowsSignin.aspx.  To get them to the sign-in page but giving them the ease of typing "intranet," you can rewrite "http://intranet/" to redirect to the signin (DNN will automatically direct them to the "real" homepage after login, which is http://intranet/home.aspx by default).

Yes, you can make it so that they do not need to login.  Even though the auth provider does allow forms authentication, but also has an authentication page that allows integrated sign in/single sign in page.  That's the Desktop Modules aspx page that I mentioned in my first page.

I would say read through the documentation, attempt an install, and see if you have any further questions.  I will admit it's not the easiest setup I've ever one, but it's also not the most difficult.