Configure Webmatrix for Windows Users

Question

I create asp .net page and i have use windows user to authenticate.

<authentication mode="Windows" />

I create simply page with one label and display information.

if (Page.User.Identity.IsAuthenticated)
{
        Label1.Text = "Authenticated = 'TRUE'<br/>";
        Label1.Text = Page.User.Identity.Name;
}
else
{
        Label1.Text = "Authenticated = 'FALSE'<br/>";
}

I often get Authenticated = 'FALSE'

How configure web.config and WebMatrix to take user from Windows?