> 
> so what would be the difference between using a Japanese OS and using an
> English OS + Japanese MUI?

http://www.microsoft.com/globaldev/reference/muizone.mspx

start here and keep reading, faq first.

---
Jpn. OS can't switch to Eng. menus and all.

Eng. OS can with Jpn. MUI installed.
---

almost 100% compatible, but not quite.  some things are still different 
and won't work quite the same under English OS + MUI.

However, it's pretty good overall, and today, you can safely use Eng. XP 
+ Jpn. MUI for just about everything and you may never encounter a problem.

Help and so forth are the common areas where they might not be displayed 
in Japanese, but Eng. instead.

---

Eng. OS + Jpn. IME only doesn't display Japanese program windows text 
correctly sometimes due to the lack of understanding of the Eng. OS w/o 
Jpn. MUI.  If you're running Jpn. programs, you really have to have Eng. 
OS + Jpn MUI or JPN XP.

---

Eng. OS + Jpn. MUI is a very nice way to go however if you want both 
interfaces.  Install XP, then MUI, then add two users - English and 
Japanese so you can pick one or the other on login.  In the Japanese 
logiin, make sure the language used, etc. (regional settings) are all 
Japanese, and voila! everything should be that convenient - just pick 
which language on login, and everything will be that way in use.

The only problem here is what happens when you install Partition Magic, 
Antivirus software, etc. - Japanese or English version?  And if English 
version, will they work on Japanese filenames?  (usually they're not 
designed for Japanese, and most companies will say you should go 
Japanese; most will work okay, but no guarentees....)  Better be ready 
because you'll have to start reading Japanesee fast once you've got the 
Japanese apps installed......

Have to be careful which language you login as before installing 
programs as well - some, such as Partition Magic, will autodetect the 
language in use, and install that version.  Watch out when you try to 
run it when logged in as another lang. user!