From MP3 to WAV is EZ. You can already do that with WinAmp. Just goto options-->preferences-->plugins-->output-->now select the Nullsoft waveout plugin. Now when u play the MP3, winamp will write a WAV into a directory u specified.
From WAV to MP3 is just as EZ. But u gotta have an encoder. I use Xing MP3 encoder.
It is also possible to to it with the Windows soundrecorder. You gotta get the MP3 codecs and stuff from the Internet, and then u can easily save MP3s as WAVs and vice versa. Tip: for MP3 music is 128 the ideal bitrate.