Obtain domain username without requiring authentication

I'm bangin' my head against the wall here.  I'm not even sure what I want to do is possible.
I have developed an ASP.NET app that will only be run on my local network (which incorporates a Windows Domain).  It's running on IIS 7.5.
I want to do this:
Dim username as string = Httpcontext.Current.User.Identity.Name
My problem is this: If I don't turn off Anonymous Authentication and ASP.NET Impersonation, I don't get the results I'm looking for.  (if Anonymous is allowed, I get an empty string returned for username, and if ASP.NET impersonation is allowed, then "username" is always whatever account is associated with impersonation, like "Network Service").  If I turn those off and then turn Windows Authentication on, it works, but only after a dialog box presents itself asking for the Windows Credentials.
What ...