There are two common file transfer protocols for Linux ISO images that guarantee the file you download is not corrupt: Bittorrent or Rsync.
Make a backup of you current corrupt ISO & then Download the bittorrent client for your choice or use rsync.
Direct either program to overwrite the exisiting current ISO. It will not delete it but will figure out which bits are corrupt & will donwload the appropriate bits. See http://www.knoppix.net/forum/viewtopic.php?t=20700