If you want to have signin as different user functionality like sharepoint in your asp.net application, following workaround you might be looking for.
After setting your web.config file's 

<authentication mode="Windows" />

and in IIS after remove anonymous authentication and enable windows authentication, have following code snippet where you want to have this functionality

In your aspx page

    <div>You are logged in as <br /> <%=User.Identity.Name%> <br /><br /> 
    <asp:LinkButton ID="lnkSignOut" runat="server" Text="Sign in as different user" onclick="lnkSignOut_Click"></asp:LinkButton>


In your code behind file

 protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        Session["logOutRequested"] = false; //Initialize value. This will be set to true when user will click on sign in as differnet user
    }
}


protected void lnkSignOut_Click(object sender, EventArgs e)
{
    if (null != Session["logOutRequested"] && !Convert.ToBoolean(Session["logOutRequested"])) // Check if user is clicking link first time
    {
        Session["logOutRequested"] = true; //Set value that user want to sign in as differnet user
        Response.StatusCode = 401;
        Response.StatusDescription = "Unauthorized";
        Response.End();
    }
    else
    {
        Session["logOutRequested"] = false; //Initialize value again after user is authenticated with differnet or same user again.
    }
}


Hope this workaround will help.

In case if above workaround is not working, you may try following one
www.roelvanlisdonk.nl/?p=825