Usually for video converting the best cheap solution (if I am not mistaking it could be free) is Virtual Dub. But Virtual Dub I don't think it will open wma, but u try it.
U have to be carrefull when choosing an encoder because the net is FULL of such kind of programs that makes u to give the money and they encode your movie at a deplorable quality.
Virtual Dub is good because he processes your movie frame by frame.