Yeah, you are missing codecs. So you can either install Perian to support several codecs in Quicktime, or install VLC (videolan), a video player that can natively play most video formats. I only use VLC as it plays almost anything, does subtitles perfectly well, it's fast and it should suit al your needs. Installing Perian makes me feel like a windows user, who has to install codecs and drivers to get stuff working 😉 The good thing with Perian though, is that it also allows you to play DivX videos in Frontrow (as Frontrow uses Quicktime). So if you're setting up a Media Center and you would like to use Frontrow as media sotware on it, Perian would be your best choice.