Do not download the file again, it is not necessary. Just use any torrent client and start downloading the same file again. When it allocates that file stop downloading it, and overwrite that file with the iso you already downloaded, after that recheck the file(every torrent client should have that). Of course before that you can always check the md5sum with this tool
Download MD5 Check 2.1 - MD5 Check was designed to be a small tool that will help you create and check MD5 checksums - Softpedia
If the md5 sum is correct then you need to provide some more details of what is wrong. If the md5 sum is not correct do what is above. I know some people have broadband and don't care to download again 6 or 7 hundred files but it's still better to download 100 or 200 than 700 right
?? Also if you download files with a torrent client then you normally don't need to check that file against md5 or sha1 sum