I dont know about the rest of you, but i've downloaded many videos from ares/limewire etc. only to try to open them and they wont play because of the lack of a codec. I finally found this great program that installs a ton of codecs and I haven't had any problems playing any media file since installing it.
BTW: its a package for vista
Free-Codecs.com :: Download Vista Codec Package 4.5.6 : With Vista Codec Package installed, you won't need to install any other codec or filter.