I feel like a bum. 0_0;
anywho, whoever installed your PC, ask them to fix it.
Likewise, I bought a Dell window XP (home edition) but when my PC broke down I made my uncle fix it. Of course he supplies me the simple need of a generated PC... NOT really what my PC originally really was.
Do this, ask the person who fix your computer to re-install your computer into it's original version. I'll bet that he install you with only Service Pack 1. In my case my uncle did the same. You must provide him the disc that came along with your PC.
Now thing, please check if your computer running as Service pack 2, if it is not, it need to be service pack 2 in order to run the Japanese script correctly.
I know you mention you having a copy, but that shouldn't be the case. Your PC must be a label as a "genuine Microsoft Office" XP computer, you can install a mini program and check if your PC up to date to even have these download run. Do that 1st okay? Chances are if it says your PC is not, then you need service pack 2... which you ALSO can download from the website!