Open up QT's preferences & press Update tab & select Find & install new 3rd party softw. Other codecs of help will be DivX go to
http://www.divxmovies.com/codec/ & also 3ivx
http://www.3ivx.com/ & go to dwnld page & look on top right of page.
Worthwhile also getting a backup player like VLC:
http://www.videolan.org/vlc/ (choose nearest mirror site)