ASP.NET

private void Page_Load(object sender, System.EventArgs e) {
    PreloadTool pt = new PreloadTool(this);
    if (!IsPostBack) { pt.StartPreload(); }

    // long running operations...

    if (!IsPostBack) { pt.StopPreload(); }
}

using System;
using System.ComponentModel;
using System.Web;
using System.Web.SessionState;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.HtmlControls;

namespace WebReports {
    /// <summary></summary>
    public class PreloadTool {
        private System.Web.UI.Page page;

        /// <summary></summary>
        /// <param name="page"></param>
        public PreloadTool(System.Web.UI.Page page) {
            this.page = page;
        }

        /// <summary>Does a full render of the page for the preload.  This will fail if your page has a server side form.</summary>
        public void StartRenderedPreload() {
            System.Text.StringBuilder sb = new System.Text.StringBuilder();
            System.IO.StringWriter writer = new System.IO.StringWriter(sb);
            System.Web.UI.HtmlTextWriter html = new HtmlTextWriter(writer);
            page.RenderControl(html);
            page.Response.Write("<div id=\"preload\">");
            page.Response.Write(sb.ToString());
            page.Response.Write("</div>");
            page.Response.Buffer = true;
            page.Response.Flush();
        }

        /// <summary></summary>
        public void StartPreload() {
            page.Response.Write("<div id=\"preload\">");
            page.Response.Write(GetPreloaderTable());
            page.Response.Write("</div>");
            page.Response.Buffer = true;
            page.Response.Flush();
        }

        /// <summary></summary>
        public void StopPreload() {
            System.Text.StringBuilder txt = new System.Text.StringBuilder();
            txt = new System.Text.StringBuilder();
            txt.Append("<script language='javascript'>\r\n");
            txt.Append("if (navigator.appName == 'Microsoft Internet Explorer' || parseInt(navigator.appVersion) > 4) {\r\n");
            txt.Append("    if (document.getElementById('preload')) {\r\n");
            txt.Append("        var lmnt = document.getElementById('preload');\r\n");
            txt.Append("        lmnt.style.visibility = 'hidden';\r\n");
            txt.Append("        lmnt.parentNode.removeChild(lmnt);\r\n");
            txt.Append("    } else if (document.layers['preload']) {\r\n");
            txt.Append("        document.layers['preload'].visibility = 'hide';\r\n");
            txt.Append("    }\r\n");
            txt.Append("}\r\n");
            txt.Append("</script>\r\n");
            page.Response.Write(txt.ToString());
        }

        /// <summary></summary>
        /// <returns></returns>
        public string GetPreloaderTable() {
            System.Text.StringBuilder start = new System.Text.StringBuilder();
            start.Append("<link rel=\"stylesheet\" href=\"Report.css\" type=\"text/css\">\r\n");

            start.Append("<table width=\"100%\" cellspacing=\"0\" cellpadding=\"0\">\r\n");
            start.Append("    <tr>\r\n");
            start.Append("        <td class=\"TitleBar\">\r\n");
            start.Append("            <img id=\"body_logo\" src=\"Logo.gif\" style=\"border-width:0px;\" />\r\n");
            start.Append("        </td>\r\n");
            start.Append("    </tr>\r\n");
            start.Append("</table>\r\n");

            start.Append("<table align=\"center\" valign=\"middle\" height=\"75%\" width=\"100%\">\r\n");
            start.Append("<tr><td align=\"center\" valign=\"bottom\">\r\n");
            start.Append("Generating report, please wait...\r\n");
            start.Append("</td></tr><tr><td align=\"center\" valign=\"top\">\r\n");
            start.Append("<img src=\"progress.gif\">\r\n");
            start.Append("</td></tr></table>");
            return start.ToString();
        }
    }
}

if (!IsPostback) {
    // Add controls
}