[KyleGoetz]

Yuusuke,

Every major OS in use right now natively supports Japanese: OSX and Windows XP (so presumably Vista does, too) come pre-bundled with Japanese. Linux can be made to support it as well.

From there, just use any word processor you like. OpenOffice is free and legal. Microsoft office is either free and illegal or unfree and legal.

I'll take free and legal, personally.