Web Forms :: Displaying A JavaScript 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.

<%@ 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.Append("/script>");
ClientScript.RegisterStartupScript(this.GetType(), "PopupSb", sb.ToString(), false);
}
</script>
<asp:Content id="contentPage" runat="server" contentPlaceHolderRoot">
// Controls not shown because I don't want too many lines
</asp:Content>