Try Virtualdub for converting files. Read up on codecs etc a bit. Assuming WMM can handle h264 (if it can't give up on it) you probably just need to change the container to an avi and NOT re-encode the video.
I'd try and find something that can edit your presumably h264 mov files natively though, as if you have to re-encode (rather than just change the container) then it'll take you HOURS before you can even start editing and you'll take a quality hit.
"Converting to an AVI" is a fairly meaningless phrase. You have a few components to a video. The container format (in this example either avi or mov), the codec (used to encode the vide) and the audio codec.
A video file's a container + video codec + audio codec. If you *must* use WMM, let us know which codecs and containers it supports.