Japanese Input Support For Western OSes?

RobM asks: "I'm currently studying Japanese, both for fun (Anime) and for profit (possible employement in Japan). To be able to better learn the language, I was planning to use my PC to read Japanese pages and then to manually translate them using one or more of the open Japanese dictionaries available. To be able to input the Kanjis I have to add to the system a Kanji input method and the related software , but none of the western distro has support for this. Moreover, there are a lot of tools for similar tasks, many of which are poorly documented and/or very old." How do the more common Operating Systems (Windows, Linux, BSD, etc.) compare when it comes to Japanese support?