The software i use is called Konvertor, which i got free from the computer weekly website a while back. It promises to convert all image, music and video files into any other format. If you can't get it through that site though, try Download.com, which is full of free software of this type
Microsoft also do a Plus! bonus pack, free on some mag covers, that includes add ons for Media Player. One of these is called MP3 Audio Converter LE (Light Edition, as it's the freebie) which does a brilliant job. Available free from their site:
http://www.microsoft.com/windows/windowsmedia/down load/bpmp3convert.aspx