Serious help needed on getting users login id from windows in domain

I have a requirement as follows:

1) create simple application in .net 4 and iis7 for users to reserv room for library discussion room

2) form will be very simple just student name, email, phone and description, when students go tohttp://mylibrary.com/request.aspx

student name, email and phone number must fill automatically without asking them to provide their credential again because they provide their credential already when they login to Library computer which are in library domain:  myLib\johnk. Once i get the  id johnk then i pass the parameter to get name, email and phone from AD.

my problem is: browser always asked the pop up form. i made <authorization mode ="windows" > in web config too. work in localhost like charm but not in the server (iis7)

my code to get users id :

String userName = HttpContext.Current.User.Identity.Name.toString();

Can anyone guide me what going wrong ?