System.IO.Stream iStream = null;

            // Buffer to read 10K bytes in chunk:
            byte[] buffer = new Byte[10240];

            // Length of the file:
            int length;

            // Total bytes to read:
            long dataToRead;

            // Identify the file to download including its path.
            string filepath  = @"E:\software\SQL Server 2000 Personal Edition.ISO";

            // Identify the file name.
            string  filename  = System.IO.Path.GetFileName(filepath);

            try
            {
                // Open the file.
                iStream = new System.IO.FileStream(filepath, System.IO.FileMode.Open,
                    System.IO.FileAccess.Read,System.IO.FileShare.Read);
                Response.Clear();

                // Total bytes to read:
                dataToRead = iStream.Length;

                long p = 0;
                if(Request.Headers["Range"]!=null)
                {
                    Response.StatusCode = 206;
                    p = long.Parse( Request.Headers["Range"].Replace("bytes=","").Replace("-",""));
                }
                if(p != 0)
                {
                    Response.AddHeader("Content-Range","bytes " + p.ToString() + "-" + ((long)(dataToRead - 1)).ToString() + "/" + dataToRead.ToString());                   
                }
                Response.AddHeader("Content-Length",((long)(dataToRead-p)).ToString());
                Response.ContentType = "application/octet-stream";
                Response.AddHeader("Content-Disposition", "attachment; filename=" + System.Web.HttpUtility.UrlEncode(Request.ContentEncoding.GetBytes(filename)));

                iStream.Position = p;
                dataToRead = dataToRead - p;
                // Read the bytes.
                while (dataToRead > 0)
                {
                    // Verify that the client is connected.
                    if (Response.IsClientConnected)
                    {
                        // Read the data in buffer.
                        length = iStream.Read(buffer, 0, 10240);

                        // Write the data to the current output stream.
                        Response.OutputStream.Write(buffer, 0, length);

                        // Flush the data to the HTML output.
                        Response.Flush();

                        buffer= new Byte[10240];
                        dataToRead = dataToRead - length;
                    }
                    else
                    {
                        //prevent infinite loop if user disconnects
                        dataToRead = -1;
                    }
                }
            }
            catch (Exception ex)
            {
                // Trap the error, if any.
                Response.Write("Error : " + ex.Message);
            }
            finally
            {
                if (iStream != null)
                {
                    //Close the file.
                    iStream.Close();
                }
                   Response.End();
            }