First go to the control panel and Pick "Date, Time, Language, and Regional Options".
Then pick "Regional and Language Options".
You should get the "Regional and Language Options" window like below. At the top click the "Languages" tab.
The first thing you need to do is click "Install files for East Asian Languages". Once you do that click "Apply".
Next click the "Advanced" tab and where it says "Code page conversion
tables" you need to check "Japanese" and click "Apply". Until you do
this Japanese will not be an option in certain places.
The next step is to set Japanese for "Language for non-Unicode
programs". This particular option will make it possible to run Japanese
software as well as type Japanese almost anywhere including filenames,
notepad, etc. Of course some programs don't support Japanese.
There are two problems with this setting though. One is aesthetic,
the default system font becomes a font that supports Japanese and in non
unicode Japanese the character / becomes ¥. That can be confusing since
all paths almost anywhere in Windows will now have that symbol instead
of \. For example C:\windows\system32 becomes c:¥windows¥system32.
The second issue is that some programs look at this setting to decide what language to install in. Two that come to mind are Apple's Quicktime and Nero CD.
In order to install them in English you need to set this setting back
to English, reboot, install, set it back to Japanese, reboot.
After you are finished rebooting get back to the "Regional and
Language Options" control panel, go back to the "Languages" tab and
click "Details…" under "Text services and input languages".
That will bring up this window. Click "Add…".
Set the input language to Japanese if you want to type Japanese.
Click "Okay" until you are out of the control panels.
Japanese for non-Unicode programs is now set.