
// get the username
User = System.Web.HttpContext.Current.User

//force the authentication to see the site - this can get real tricky
<configuration>
    <system.web>
        <authentication mode="Windows" />
         <authorization>
             <deny users="?"/>
          </authorization>
    </system.web>
</configuration>

link for windows authentication stuff
http://weblogs.asp.net/scottgu/archive/2006/07/12/Recipe_3A00_-Enabling-Windows-Authentication-within-an-Intranet-ASP.NET-Web-application.aspx

// this needs to be run after .Net4 is loaded
C:\Windows\Microsoft.NET\Framework64\v4.0.30319\aspnet_regiis -i