Server Logon displayed

I just added some code to my app.
            System.Security.Principal.WindowsPrincipal p = System.Threading.Thread.CurrentPrincipal as System.Security.Principal.WindowsPrincipal;
            string strName = p.Identity.Name;
I had to disable anonymous access for this to show. On my machine, I am now fine. However on another user's machine accessing the app brings up the server logon. Can anyone help?
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