Authentication

Hi experts,here is my issue

 I need to test my website now , so i copy it to a server running Windows server 2003 , I have already installed .NET framework 2.0 , My Authentication technique is to get the username and see if it is exists in the domain using this line of code

System.Security.Principal.WindowsIdentity.GetCurrent().Name.ToString();

On the development enviroment this works fine , but when I moved to a production environment the authentication always failed because the user I get is "NT AUTHORITY\NETWORK SERVICE".

My questions;

1. What should be the authentication mode in my web.config "Windows" OR "Forms" ?

2. Is that line of code is the right way t ...