From what I've read new versions of Movie Maker actually support AVCHD .m2ts files, so you shouldn't need to recode. If it can't read the files I'd simply suggest you use another editor, because that format is pretty common and any decent NLE – which I honestly don't think Movie Maker is – should be able to handle it.
You might be able to get more support for the old Windows Movie Maker versions with the ffdshow tryouts codec library. Windows Movie Maker seems to have an option to enable them (after you've installed them) in the compatibility settings:
All you need to do is rename your file so that it has the file extension .avi. Using this trick you should be able to import FLV, MP4, and other file types. Don't forget to rename your files back to their original extensions when you are done using them in Movie Maker. Renaming may not always be necessary. You should always first try to import using the original filename!
Another configuration tweak you you might need to make is to adjust the compatibility settings of Movie Maker.
Movie Maker menu -> Tools -> Options -> Compatibility -> enable "ffdshow video decoder" (and other audio/video decoders from the codec pack that may be listed there)
The main problem is: you can't create WMV 9 (or SMPTE VC-1) video with
ffmpeg. Only WMV 7 and 8 are supported, but I would generally not recommend using those, as their visual quality isn't the best compared to most competitors.
You can also try creating a high bitrate MPEG-2 intermediary file or so, but this is a far from ideal solution:
ffmpeg -i input -c:v mpeg2video -b:v 10M -c:a pcm_s16le output.mpeg