Duff-Man says....if you just want to play the song (ie - not edit or anything) you can use Garageband's "export to iTunes" feature (it is in the File menu). It is then exported as an aiff file but from iTunes you can then convert to mp3 or aac and transfer to the pc however you wish....oh yeah!
Duff-Man says....once you have exported inti iTunes, select the song from within your iTunes library and then go to the "Advanced" menu where you should see the second item in the menu is "convert to aac" or "convert to mp3" depending on what your iTunes preferences are set to.....oh yeah!