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",