Alert display on top of Modal Popup asp.net

Question

I'm wondering how, if possible, to display an alertbox over a modal pop. I'm using the modal popup extender to display a form which the user fills out. On the form there are fields that the user must fill out, if they don't fill out a field and click submit then I want to display an alert box. As it is right now, I can't get the alertbox to display at all while the modalpopup is still active. Here is the code for the alertbox I'm trying to use:

   Page.ClientScript.RegisterStartupScript(this.GetType(), "error", "alert('Please coomplete all of the required fields');", true);

Answer 1

 string Msg = "Your Message";
            Response.Write(@"<script language='javascript'>alert('" + Msg + " .');</script>");

It works fine for me