I do (also sometimes) use Emacs on Windows, but I must admit I have little experience of the issues surrounding Japanese textual input (I've not checked but I would have automaticaly assumed "there's an Emacs mode for that", but perhaps not) - without that additional requirement just a plain old basic Emacs would have worked out of the box, no?
But anyway, good luck in your travels in scratching that itch! :)
But anyway, good luck in your travels in scratching that itch! :)