I don't have Win7 but if it is like any other Microsoft OS, they won't include a decent MP3 encoder. For obvious reasons, they want you to use WMA.

Unless you want to mess with trying to install a better MP3 encoder within Windows, you may want to output your song to a WAV file and then use any number of WAV to MP3 converters available. Many people here use Audacity but you still need to install an MP3 encoder. If you choose this, HERE are the instructions for doing the one-time procedure. Any problems you have converting can probably easily be answered here.

Some media players have this ability. My choice is Foobar2000 found HERE.

Stick with the LAME encoder. Open-sourced and well supported. If you don't want to learn Audacity, the LAME website has plenty of links to front-end programs to help you with the conversion. Their site is HERE.

Good luck