Log in as a different user

 I have a website using Windows Integrated Security.
The user will be logged in using the credentails they supplied when logging into the machine.
However, the machine may be used by many different users and may remain logged in by the same user.
The website has a link -  <UserName> - Not <Username>? Login. This should display the windows login box so they can log in as a different user to that logged on to the machine.
I have the following code but although the login box appears the new user isn't logged in and the original user remains logged in. Can anyone help with this please?
 Protected Sub ChangeUser_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles ChangeUser.Click
        Session.Add("ReAuthenticate", True)
     &nbs ...