I bought some songs from an online music store that comes in .m4a file.
Unfortunately, this is a proprietary file format and windows explorer cannot
display the tags (artist, album, etc).

Is there another media file type that can act as a container for AAC/m4a
file, AND has tags properties that windows can edit and display?

The last resort would be to reencode the m4a file into wma or mp3 file. This
would allow me to tag the file but would also degrade the song due to
transcoding loss.

I'm using vista. I'll have to test windows 7 to see if it can tag .m4a file.