converting a .wmv file to .avi

i've put together a short piece in PhotoStory, which i believe is in a .wmv
format. the disc i've burned is supposed to play in standard DVD players, but
mostly does not (with one exception), nor will it play on a mac. one solution
i've heard is to convert the .wmv file to an .avi file, which would be a more
common format, and thus viewable on mac, and possibly on DVD players.

true? if so, how can i do this? would i use moviemaker? and if so, how?
(i've looked, and see i can convert .avi's to .wmv's, but not vice-versa)

thanks for any tips