CODECs is the word that you're missing. Might I recommend you get a backup player with in-built codecs such as VLC (not a pretty player but it does the job) which will play all sorts, & dvd's & vcd's & online streaming of audio & video. Get it here: http://www.videolan.org/vlc/ (choose nearest mirror site)

For your other players, I'd suggest GSpot which will identify which codec you need to get. See CODECs list and alternate players for Movies