// типа (А)
        //String URL = @"http://mssql2012/ReportServer/Pages/ReportViewer.aspx?/rpt_NewWind01/rpt_Prm1Gap&rs:Command=Render&ContactName=ann";

         // типа (Б)
        String URL = @"http://mssql2012/ReportServer/Pages/ReportViewer.aspx?/rpt_NewWind01/rpt_Prm1Gap&rs:Format=EXCEL&rs:Command=Render&ContactName=ann";


        String m_filePath = @"C:\Users\Desktop\SRSR 2012\download3.xls";


      protected void bt_RptExcel_Click(object sender, EventArgs e)
        {

            HttpWebRequest request = null;
            HttpWebResponse response = null;

                request = (HttpWebRequest)WebRequest.Create(URL);

                request.Timeout = 100000;

                request.AllowWriteStreamBuffering = false;
                request.PreAuthenticate = true;
                request.Credentials = new NetworkCredential("login", "password");

                response = (HttpWebResponse)request.GetResponse(); // <<= тип (Б) генерит ошибку "The remote server returned an error: (401) Unauthorized."
                Stream s = response.GetResponseStream();

                //Write to disk

                FileStream fs = new FileStream(m_filePath, FileMode.Create);

                byte[] read = new byte[256];

                int count = s.Read(read, 0, read.Length);

                while (count > 0)
                {
                    fs.Write(read, 0, count);
                    count = s.Read(read, 0, read.Length);
                }

                //Close everything

                fs.Close();
                s.Close();
                response.Close();

        }