You don't have to do that. Go to Control Panel->Regional and Language Options->Languages. Select Install files for East Asian langauges, press apply. It may ask for your Windows XP disc, and it may prompt you to reboot when finished. Next, you need to add the keyboard to by pressing "Details" on the same screen, then add the Japanese keyboard.
Why don't you just change IME over to hiragana? I have an English OS and english keyboard and have no problems. IME is your friend.
One is tempted to define man as a rational animal who always loses his temper when he is called upon to act in accordance with the dictates of reason. - Oscar Wilde