This doesn't actually seem to be an Xtorrent issue, nor the file downloaded itself.
Your issue seem to be with codecs, how the computer is interpreting the data.
Download and install Perian http://perian.org/
. Perian is a codec package for Quicktime which will interpret the .avi file correctly. (Same functionality as K-lite Codec for Windows.. just that it is for the Mac)
Hope that fixes your problem