The programs listed on this page all run on Windows XP, and presumably on Vista/7. Mac and Linux software may be added in the future.
JWPce – a free Japanese word processor that includes its own IME (input method editor), allowing you to create documents in Japanese even without Japanese language support installed (though you should try that first). Can also be run from a flash drive. The website is hard to navigate, but you should find a download link at the bottom of the main page.