Use Microsoft IME.
Here's how
1.Go to control panel and right under Clock, Language and Region section, click change keyboards and other input methods
2.A dialog box will pop up and click change keyboards
3.On the general tab there is an Add button in the Installed Services box. Click it.
4. You will see a list of languages and find Japanese. Click the + button and under Keyboard, there will be a check box saying Microsoft IME. Put a check on it then hit OK. You'll now return to the general tab and hit Apply then OK.
5. Now in your taskbar you should now see the language bar. At default, it should say EN (for English keyboard) . Just click on it and select JP and choose an Input Mode for it (Hiragana, Katakana and so on)
You can now type in Japanese