How to convert .m4a into .avi?

I recorded some videos on my nokia 5800 and I want to edit them with windows movie maker but it can't open the file. turns out nothing on my computer other than quicktime can open it, my quicktime can't convert files though. my itunes can open it too, but it became audio only. I searched for some converter on the net and I can only find m4v to mp3 and wav. ._.

the extention is .m4a MPEG-4 audio

please help
sorry for my broken english