Since you're using Quicktime, I assume you're on a Mac. Quicktime's default decoder for a number of video types is pretty broken. Try installing the Perian QT component
and playing the videos again. You could also try using a different player, like VLC
or MPlayer OSX Extended
If you're on Windows, you should get the Combined Community Codec Pack
and play the videos with the included Media Player Classic Homecinema.