following is the code for file download window
const string fName = @"C:\picture.bmp";
FileInfo fi = new FileInfo(fName);
long sz = fi.Length;
Response.ClearContent();
Response.ContentType = MimeType(Path.GetExtension(fName));
Response.AddHeader("Content-Disposition", string.Format("attachment; filename = {0}",System.IO.Path.GetFileName(fName)));
Response.AddHeader("Content-Length", sz.ToString("F0"));
Response.TransmitFile(fName);
Response.End();