How to get user name in Windows Authentication

Hi,
        I am working on a web application with windows authentication mode. I set it in web.config...
 <authentication mode="Windows"></authentication>     <identity impersonate="false"></identity>
When I run application, it ask for user name and password, I put my system username and password, and It get me logged in successfully.
I am trying to access current username in my application, but I am not getting what I entered, it gave me NT AUTHORITY\SYSTEM as user name.
One more thing, If i run application through Visual Studio, it gives me correct user name, but through IIS, I browse it, it does not give correct user name.
Below is the code that I have written to get current user name..
System.Security.Principal.WindowsIdentity.GetCurrent().Name
But it's not giving my system user name.
I am using applica ...