OK peoples, if you are really good at computers, itunes, and music formats then you need to answer this.?

ok. so i have a creative Zen Vision M and it has all the features of an ipod. however, since my computer's music is all in MPEG-4 a.k.a. M4a, i cannot transfer my music because My Zen tells me that it does not support M4a/ MPEG-4 files. Is there a way to somehow change the format of my files such as M4a/ MPEG-4 to mp3 or WMA? If you know the answer to this question, please answer it.