I have recently repaired my corrupted "ubuntu-12.04-desktop-i386.iso" file, which I downloaded with free Internet Download Manager (IDM). The md5sum of this file was not matching with the correct one and when I tried to boot using a live usb it said something like "corrupted kernel image". I did some googling and finally repaired the file using torrent as follows:-
I never used torrent before so I first downloaded a torrent client (uTorrent) and got a bit familiar to it. Then i found a torrent file of the exact iso. Afterwards I
put the corrupted iso image in the location* where the file was about to be downloaded by torrent, BEFORE STARTING THE TORRENT DOWNLOAD for the file. And when I started the torrent download then instead of starting the file download it checked the corrupted file kept at the location by me and downloaded 10 or 15 MB of corrupted data and when I checked the md5sum with Cygwin it matched with the correct one. I was saved from re-downloading the 701 MB file on my costly 3G data pack.
P.S: this location can be easily known by starting the client from the torrent file and looking at the location it prompts to save the file