Go My Computer => Control Panel => Regional and Language Options =>Languages
Then
*select 'install files for east Asian languages'
*Click ok/apply and it should install
*Go back into Regional & Language options - Languages
*Click 'details' under where it says text services & input language
*Click 'add' and add Japanese.
Then there should be this little box on the blue bar at the bottom you can use to switch languages.
Hopefully this should work.