Deploying asp.net application in IIS6.0 with Forms authentication mode

Hi,

I am getting user name by using the following code:

txtName.Text = Environment.UserName;

I have deployed the code in IIS and while running the application then i am getting
user name as "NETWORK SERVICE" in the textbox instead of logged user name.

And I am using the FORMS authentication mode. (this is mandatory)

Thanks in advance