As we do not currently have much direct experience dealing with DOS, Windows, NT or Linux, we are pointing people at the University of Washington's Japanese Computing on DOS and Windows page for detailed information.
There is also a great deal of useful information at TN's Using Japanese with a PC... page.
A simple solution which does allow for display of Japanese text on an otherwise non-Japanese capable machine (but which does not, to our knowledge, allow you to type in Japanese in forms, etc.) is available for Windows users browsing with Internet Explorer.
PC users running Windows 95 and browsing the web with Internet Explorer 3.0 can no longer upload a patch from any obvious site at Microsoft. Perhaps this is because all support may be aimed at IE4. This patch used to enable Japanese text viewing inside of their browser.
However, Internet Explorer 4.0 users should look for the multilanguage add-on at http://www.microsoft.com/ie/ie40/download/addon.htm. (Just follow the link, and then choose the OS link from the page and follow the directions from there.)