getting domain user name without windows prompt using c#

I have an asp.net c# site that I have setup in IIS 7.
In IIS I have enabled basic and windows authentication so I can get the domain users username, so if there is a way to get this without prompting that'd be a perfect solution for me.
I am using the follwing statement to get username:
String username = System.Web.HttpContext.Current.User.Identity.Name;
lblUserName.Text = "Welcome" + username;
I want to avoid windows prompting for login credentials.
Can you please give me suggestions how can i do this?
Thanks,
Venkata.
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