The challenge in Japan is to find a reputable dealer that can sell you a computer with an operating system in English. Obviously, the major market share in Japan is for Japanese and consequently the Japanese OS is standard.
There are places where you can buy computers with an English OS. We list a few here and expect this list to grow as people find new places and as more and more stores cater to the English speaking population in Japan.
Here is a list of a few stores>> Most are online sites, but some have actual store locations you can visit.
Toshiba Direct will ship English OS to Japanese Addresses http://www3.toshiba.co.jp/jdirect/eng/
Dos Paradise - otherwise known as Prime PC has a store in OSU Kannon, Nagoya (across from Ameyoko Building) and will build you an English OS machine on request or ship you one http://www.primepc.jp/goods_pc/goods_pc.php
LAOX in Akihabara, Tokyo (just a short walk from Akihabara Station) will sell you an English OS PC at duty free prices if you have a tourist visa. If you are a resident in Japan, you must pay Japanese sales tax. http://www.laox.co.jp/store/df/index.html
A Macintosh Computer bought in Japan will be all Japanese out of the box, *BUT* can be changed over completely to English in about 30 seconds. A bought-in-Japan iBook or PowerBook will come with a Japanese keyboard, but one can order an English keyboard instead for no extra charge. See any Mac Dealer. http://www.apple.com/jp/retail/