@prh99: MPC does work, however VLC can indeed use external codecs.
@Andres: I would NEVER install that. You don't need all those codecs and they just cause conflicts and problems. I've seen so many people that have installed those packs only to have to uninstall them because some game or video screwed up.
You only really need at most Real Media Alternative, Quicktime Alternative, DIVX, xvid and then maybe AC3 codec. Even then really on a 'as needed' bases if you use
VLC since it can play a lot of stuff without codecs . If a video doesn't work, use
GSpot to identify what codec is required. You'll save yourself a lot of headache if you install each codec individually and not lumped together.