You can use a tool called G-Spot (oo-er :naughty) to determine which particular codec a film you're trying to view actually requires, and then you can download that particular one.
As bmxbandit says, I'd avoid codec packs if poss... I too have had problems in the past using one of these.
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Here's the URL.... I had to be careful what keywords to use in Google
http://www.headbands.com/gspot/