There is a thread on this forum here:
http://www.japanforum.com/forum/japa...ge-help/21099-[guide]-read-write-japanese-your-windows-xp-pc.html
on how to do it on windows XP, i don't know about vista though
Edit: Ok so the link doesn't work properly on these forums because of the square brackets in the URL, so just go to the Japanese language help part of the forum and its a sticky thread in there.
Just remember you'll need a windows XP disc, unless anybody knows how to install it without one.