If you have XP, Vista, or Windows 7, there should be this thing called a Language Toolbar. Here's how to find it:
1. Go to the Start menu and open the Control Panel
2. Click on "Clock, Language, and Region"
3. Under "Regional and Language Options," click on "Change keyboards or other input methods."
4. Under the "Keyboards and Languages" tab, click "Change keyboards..."
5. Under the "General" tab, click "Add..." and choose the languages in which you want to type
6. Under the "Language Bar" tab, click the bubble that says "Docked in the taskbar"
7. Click "Apply" and close out of the windows when it finishes applying
When you do that, there should be a small area on your taskbar that says "EN," which stands for English. Click on it and a pop-up menu will appear with your selected languages. There are only some programs that can use the foreign-language keyboards, and Microsoft Word is one of them.
When using the Japanese mode, click on the icon that looks like a skinny letter "A." This determines the input mode, of which you can choose Hiragana, Katakana, and Half-Width Katakana. When you use the Hiragana mode, the computer will automatically translate what you type into kanji, if any is applicable. You can change which kanji is used by highlighting the word, right-clicking, and choosing the correct kanji from the drop-down menu.