Probably the files you've been trying to d/load are fakes.

See Recognising Fake files, also this post by LOTR: http://www.gnutellaforums.com/showthread.php?t=63993.

Check files with Bitzi Web Lookup before downloading. It'll help you sort out if it's a good file or not. While on the Bitzi site, search for the file there and use the magnet link to launch the file in limewire. Then search for it again in LW. Sometimes that'll help you find the good files.
Also preview the file when it gets to 10% to check that it's what the title says.

See this thread for tips on searching and downloading files. How to find music(same principles apply to movies).

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