Summarize the following:
If you have Windows XP or 2000, you do not need to download the Language Pack; instead, install the built-in version on your computer according to the corresponding instructions above. Click Continue to begin validation process. When you are done, click Continue to return to the language pack installation.  This should install it automatically.

Summary:
Go to the external link Japanese Language Pack for Microsoft Office XP provided below. Validate your Windows Office edition (if necessary). Download imejpn.exe and follow the instructions. When the download is complete, double-click the file to run it. Reboot your computer.