How to handle WWW-Authenticate header in asp.net code

Hi I am trying to implement "Sign In as a different user" functionaly which is similar to sharepoint implementation in my website

I can able to produce the password dialogue box by using the following code but don't know how to proceed

        Response.Clear()
        Response.Buffer = True
        Response.Status = "401 Unauthorized"
        Response.AddHeader("WWW-Authenticate", "NTLM")
        Response.End()

the else part in the following code never works. I tried to replace with session variable instead of viewstate with no remedy.

Anybody shed some light on this please. Remember in my web site the users are already authenticated with windows integrated authentication (anonymous disabled) and I just wanna to impl ...