Displaying a Java Script popup box in front of the page after it has been rendered

I have a Java Script popup messae box that is displayed when an application user first displays
the web page. Everything works great, but when the popup displays the background of the screen
is white, with the popup in front of it. Instead I would like for the popup to be displayed in
front of the web page after it has been rendered with all of the controls on it.
Is this possible?
<%@ Page Language="C#" codebehind="Default.aspx.cs" inherits="Colso.Site.Default" masterpagefile="~/Master.Master">
<script runat="server">
protected void Page_Load(object sender, EventArgs e)
{
StringBuilder sb = new StringBuilder();
    sb.Append("<script language='JavaScript'>");
    sb.Append("alert('message to client')");
    sb.Append("<");
    sb.A ...