how to show popup when downloading the excel file

i am exporting the gridview data to excel.

before it downloading i want to show the popup. for that how can i do using C#

My code as follows with out popup code

System.Threading.Thread.Sleep(3000);

System.IO.StringWriter sw = new System.IO.StringWriter();

HtmlTextWriter htw = new HtmlTextWriter(sw);

gvEmpdetails.AllowPaging = false;

BindGrid();

//Applying stlye to gridview header cells

for (int i = 0; i < gvEmpdetails.HeaderRow.Cells.Count; i++)

gvEmpdetails.HeaderRow.Cells[i].Style.Add("background-color", "#df5015");

gvEmpdetails.RenderControl(htw);

Session["ResponseData"] = sw.ToString();

iframe1.Attributes.Add("src", "Download.aspx");

Response.ClearContent();

Response.Buffer = true;

Response.AddHeader("content-disposition", string.Format("attachment; filename={0}", "Employee.xls"));

Response.ContentType = "application/ms-excel";

string responseString = Session["ResponseData"] + "";

Response.Write(responseString);

Response.End();

from the above i want to add popup code before it is downloading to excel.

for that how can i do in asp.net using C#.