its almost certain that you hav ea virus that is trying to infect the torrent, this is making utorrent think that teh last packet it corrupt, this makes it recheck the file, it then realises it cannot get exclusive access to it and crashed out. im almost certain this is a virus, maybe spyware.
don't know what your running but get spybot Seek and Destroy
it will check for spyware, as for virus killers, you probably have one but there's a trial of kaspersky
about, use it, then go back to something like MSE
. if you dont want to pay for it. (free antivirus is almost as good anyway)
also there is an option in Utorrent that lets you append the file extention of incompeate files, this makes it clear what has and hasnot finished, it also makes the files invisible to most spyware as its an unknown file type (makes then filename.!ut)
i hope this helps, it it turns out not to be any of this stuff do let me know what it was in the end :P