Control Panel -> Regional and Language Options.
Hit the Languages tab.
Check "Install files for East Asian languages" and press Apply.
(You need to do this first, so you have the files for the keyboard install.)
In the same window click on Details, then in the popup, choose Add.
Choose Japanese, then click OK.
(You might have to restart here or at the step before, I forget.)
When all that's installed, you'll get a little EN button on the bottom right.
Click it and switch to Japanese.
Right-click it and choose "Adjust the Language band position".
Click on the button with the A, and choose Hiragana.
(You'll have to do all of this for every program you want to type Japanese in.)
Now, just type stuff. It'll appear in hiragana, but when you press Space, it'll change to kanji/kana. Use Left and Right and Space to change stuff that it's got wrong.