Audio Codec problem?

Yea well i like to download videos online and such and recently i ve encoountered some issues about not being able to play or hear the content... for example, a video plays on wmp and it says "downloading codec", "error", "codec not found" so the vido plays but no sound... also i found my computer which has a dvd-rom drive, cant plays dvds, no "encoder apparently"... any idea where i can get the decoders and codecs??? i have quicktime, latest wmp version, and latest divX... thanks.