Replying to Help! Computer problem/movie files (mac)

So I downloaded a bunch of movies from some Torrents.. and they are .avi files. I have flip4mac.. so it should play in quicktime, i think.. If i try to play the movies.. it either tells me "The movie file could not be opened".. or it will play in quicktime, but there will be no picture nor sound... just a blank white screen.. for two hours..(the length of the movie)..

So am i missing some kind of update.. or I don't know. Or maybe another movie player will work better... or maybe conicendentally, all 3 movies I downloaded are screwed up?

Thanks alot for any help.