string fileName = BuildFile();

            Response.Cache.SetCacheability(HttpCacheability.NoCache);
            Response.Clear();

            string FileName0 = String.Format("{0}.xls", Guid.NewGuid());

            Response.AppendHeader("Content-Disposition", String.Format("inline; filename=\"{0}\"", FileName0));
            
            Response.ContentType = "application/vnd.ms-excel";

            byte[] data = File.ReadAllBytes(fileName);
            File.Delete(fileName);

            Stream str = Response.OutputStream;
            str.Write(data, 0, data.Length);
            Response.Flush();

 Page.RegisterClientScriptBlock("BLANK", "<script>window.open('../FilterForm/FilterExcel.aspx?ZAP=1&REP=" +param + "');</script>");

<base target="_blank" />