Corion's advice is sound (pun unintentional :)), but if you wanted to try the 'full' approach, you could have a look at a combination of LAME and Audio::MPEG. There are LAME binaries for Windows available, and Audio::MPEG's author seems fairly confident that it *should* work, although he hasn't tested on anything 'cept Linux.
Included in the Audio::MPEG docs is an MP3->WAV routine which may come in handy.