Well I use QT Pro so I have a few xtra codecs anyway. But go to your QT Preferences, Select the Plug-ins Tab, then press the MIME button & tick all options. You may also need to downld a few more.

If you want to try a reliable player that has built in plug-ins & 'plays everything' except the odd wmv file then try VLC: http://www.videolan.org/vlc/ (choose nearest mirror site)

Another alternative is MPlayer (click on link) which is equivalent.

Oh by the way if you want to know how to make VLC the default app to launch video files from LW just ask.