Do you use XP?
Control Panel -> Regional and Language Options -> Language Tab -> Details -> Settings Tab -> Search for Japanese (You might need your XP CD) then Add.
You might want to check if your language bar is being showed, you can do this by clicking the 'Language Bar' in the same window.
You should see a weird looking bar near your system tray, just play around with it you should be able to type in Hiragana, Katakana and Kanji with it.
Hope that helps.