It reminds me of me when I encountered the very same message and started looking all over the net for a solution (I checked this place on a whim and found the answer here...). Ah the memories...
But I digress...
Click here:
VLC and dld and install VLC. It will play most everything (except .avi which contain WMV3 but AFAIK there is no known player which plays that kind of .avi for Macs
)
Also get this:
Mplayer is another nifty player. It's interface isn't very good IMO but it's codec support is better than VLC (I mainly use VLC though)
These should get you by.
