I use Any Video Convertor
and although it cant output m4v (which a just an mp4 variant) I have just happily converted a freshly downloaded m4v to wmv with no problems or loss of quality, it does keep offering it's professional edition but still works perfectly in the freeware version without limitation. I recall it used to be ad based but since the last update I havent noticed adds (maybe due to my system config or 3rd party apps I have installed).
Anyway does the job!
P.S. just noticed it supports manual adding of output codecs if required, also supports multithreading in freeware edition.