It might be a QuickTime video plug-in codec issue, http://www.gnutellaforums.com/292880-post4.html
However, if you are downloading torrents, make sure it's the actual video you open with QT Player (or other player) & not the torrent file you downloaded from the website. ie: check the video file size. Torrent files will be perhaps 5-30 KB, whereas video files will be perhaps 100 MB - 2+ GB. If you downloaded a dvd with it's video folder & files inside, then open it using apple's dvd player.