I have long been running Japanese applications on an English version of Windows XP on my old self-made PC. I built that sucker in early 2003 and it's still going strong. In fact that's an understatement, it's actually my main work machine! I may have had various problems at the beginning with Japanese apps, but have not had any in nearly 6 years. So I had forgotten what it was like.
Anyway, my new Dell Latitude E6500 notebook came with an English XP. I installed Japanese Buffalo wireless client software so it can talk to my Buffalo router, and this software displayed a lot of garbage characters. Later I installed Hidemaru and had the same problem, lots of garbage characters.
I poked around and found the solution:
Control Panel -> Regional and Language Options -> Advanced -> Language for non-Unicode programs
I set this to Japanese, and this solved the problem.