Redirect To Action Without Changing URL MVC

Hi
dudes i am using mvc3 , i have create application level event for exception of errors as below,

protected void Application_Error(object sender, EventArgs e)
{
Response.Redirect("~/PageNotFound/Custom404");

}

suppose i am typing url (localhost:23/xyz/abc/test(what ever)
it is showing custom404 page , but i want show this page but without change url can any one help me
please.