VS 2005 - Intranet Windows User Name

I have been tasked with creating an intranet site and have been able to do everything so far (leaves the door open wide, I know ) EXCEPT get the username that the person is logged in as under their own computer.

<system.web>
<pages styleSheetTheme="Black"/>
<customErrors mode="Off"/>
<compilation debug="true" urlLinePragmas="true"/>
<authentication mode="Windows" />
<identity impersonate="true"/>
<globalization requestEncoding="utf-8" responseEncoding="utf-8"/>
</system.web>
Under my master page I am doing this:

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
If Environment.UserName.Length > 0 Then
username.Visible = True
username.Text = Environment.UserName
Else
username.Visible = False
End If
End Sub

All I am getting is the ASP account name....what am I doing wrong??