Do a Google search for "cDex"...it's a free program that will let you convert to MP3...
Once that is installed and running, go to Options -> Settings, set LAME as the MP3 encoder, and then set the Bitrate to 128KBPS, although I would recommend 192.
128KBPS is roughly 1MB per minute of audio but it has a noticeable effect on the quality...
Then drag & drop the WAV file onto the white part on the main window and click "Encode"...MP3 files will be placed in \My Documents\My Music\MP3 I think...