Yes, you can do it on one drive.
To do this you need two partitions on your harddrive into which to install the Japanese version of XP and the English version respectively. It is called a
Dual Boot Setup.
The problem is that, at present, you have only one single partition encompassing your entire harddrive, into which the current English version of XP is installed.
Problem: you did not spec your computer or manufacture. IF you have an older computer, and IF you have an independent Windows XP disk, things are easier. IF you have an newer computer, and you have only a restore disk, or worse yet, a "hidden partition" which houses the restore program for XP, you have a problem, because if you alter the partitions on the disk you will lose your restore partition and your English version of XP and you are up a creek without a paddle.
SO-to do this you need:
> An independent version of Windows XP English;
> An independent version of Windows XP Japanese;
You first wipe your entire harddrive by removing all partitions in the disk formatting screen;
You set two partitions using one of the system disks; you target the first partition, format that partition and install XP into it;
After XP is up and booting and running good, while it is still running, you insert the other language XP disk and it will auto-boot; dial over to the hardddrive formatting screen again, target the
second unformatted raw partition, and format and install the other language XP into that second partiton;
XP automatically creates a dual-boot MBR when this is done.
Now, when you boot your computer, you will be given a choice of booting into English XP or Japanese XP; each XP will run totally independent of the other, even though they are installed on the single harddrive.
There are 3rd party programs such as Partition Magic, that will supposedly create an "on the fly" partition for you without wiping your entire harddrive. This in theory will preserve your present installation of XP, and create an alternate partition for you to install Japanese into. However:
* Practice on a different computer first!! I cannot even begin to count the number of folks that have trashed their operating systems attempting this, especially the first time!
* Even in the unlikely event that you succeed, you are only postponing the inevitable. You will still have a "hidden partition" that will (probably) be unusable if you ever have to restore XP in the event of a catastrophe. And if you ever lose your English version, you will still have to purchase a new English standalone version.
Hope this helps. If you have additional questions don't hesitate to post back!