Probably the files you've been trying to d/load are fakes. See
Recognising Fake files.
Check files with
Bitzi Web Lookup before downloading. It'll help you sort out if it's a good file or not. 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).