Security :: 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:

[Code]....

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 I'm trying to do is NOT require the users to log in. Since they've already logged into the domain when they signed on their computer, I'd like to just allow them access to the ASP.NET application I've written, and identify who they are (based on their previous Windows login) using the line of code above. I don't need them to re-authenticate, because they're already authenticated from their domain login.I can't figure out what I'm doing wrong, or how to accomplish this. It seems the closest I've gotten to what I want so far, is to turn off all authentication EXCEPT for Windows Authentication. It works then, but only if the user first types in his/her username and password again in a dialog box. Is there any way to avoid having them type in their username and password again, but still be able to identify their domain username?