Stripe,
You need to obtain the MUI (Multilingual User Interface) Japanese, in order to change your XP into a Japanese version so that you can read, type, and work in a Japanese Windows Environment.

More about the MUI:
Read about it here

THis is hard to get ahold of and I managed to get it. It is a hefty 500+ MB, and adds about a gig to your partition when you install it. It overwrites nearly 97% of the files on your XP installation.. allowing for nearly a 97% translation, some things never translate and some translate poorly, but overall it is not too bad.

I only have a link for the MUI version for windows Xp embedded and it is only 114Mb or so.
Download the MUI for WIndows XP Embedded

I don't know if it works for the regular windows XP edition or not.

MOre information about the MUI and the place where I got the above link

Hope that helps.
BB