In all honesty I'm not sure - there might be a work around, but I don't think so.
There are three problems here. If you plug in a keyboard to a laptop it disables the laptop keyboard as far as I know - (anyone else out there please correct me if I'm wrong) - the other problem is that the keyboard driver gets loaded very early in the boot process and you can't choose it with a user setting - it's already loaded. Finally, if you want Japanese menus in Windows itself they need to come from somewhere - they are not built in to either the normal XP - or the IME
One solution is a dual boot situation with one OS being the Japanese XP - (that's what I have done in the past with my Japanese wife but I just used the Japanese keyboard with the English OS - Windows ME in both cases) With XP it should work as well - but it will cost you the price of an extra copy of XP, which you will have to get from Japan. If you choose this option then make sure that the Japanese keyboard is connected when you install the Japanese OS as it has to find the keyboard you want to use to install the correct driver.
The second solution - which I now use - is to have two computers - one installed fully in Japanese with keyboard as well - and then network them to be able to share files and printers etc. That works fine in XP.
As far as I know Microsoft does have an XP version that will allow different users to log on in different languages - but it's not available to normal retail users - only large companies - I believe it comes on about 14 or 15 CD roms so is really not a practical alternative - I think it's called XP Embedded.
The main point is that the IME allows you to input and read Japanese charachters - but the menus don't change, they stay English or whatever the language version is. The only program you can change is Internet Explorer - that you can change to Japanese menus as well (or any other language) with a download from the MS update site.
One last point - with East Asian support installed in XP you can install Japanese software or hardware in an English environment with no problem - before with ME I had problems with some Japanese drivers and hardware configuration programs - the Japanese script came out as garbage!
I hope that this is some help - believe me - I know what you're going through - been there myself!
Best of luck -
Kim Leece.