You need to download an fraunhofer's mp3 codec. Go to start,control panel, sounds and audio devices (classic view), click hardware, then highlight audio codecs, then click properties, then click the properties tag. This is the list of the audio codecs if fraunhofers IS Mpeg Layer 3 isn't listed then you need to download the codec.
Edit: there's a link somewhere on this forum that leads to the download its kind of a pain to find.
EDIT2: found the link MP3 Codec