All of you.. shame.
The easiest solution is to just get something has *almost* every kind of codec possible (within reason of course)..
The only media player that has almost all codecs is the infamous 'VLC' or VideoLan Player. Just Google it, and download VLC. Install it, and open the file with VLC. It supports almost ALL media codecs. If WMP or DivX or any other player won't work.. chances are that VLC will.
(It supports the most codecs of any media player, with just the normal install.. of course, you can always add 'new' codecs later, like when a new mp5 codec comes along.. xD)
~~Azurith
VLC >>>>>> ALL other media players >>>>>> WMP >>> Quicktime