Gr☺uch☺: I'm not sure (I don't use the latest version of Windows Media Player) - but my guess is that it is impossible for WMP to have every codec out there - so I would guess that even with the latest, you may sooner or later run into something that needs a codec.

... and I'm far from an expert - I could be wrong in all of this - if so, I'm sure someone will correct me.