The following links give an excellent step-by-step overview of how to setup a non-Japanese Windows system to be able to display Japanese text and how you can enter it yourself.
Installing Japanese support
Writing in Japanese
Installing Japanese support on Windows Vista
One thing I don't think they talk about in detail that's really useful is that you can still type in English even when you have Japanese selected as your input langauge. The Japanese IME (Input Method Editor) has several modes, the 2 we're most interested in are Direct Input (not to be confused with the DirectX technology) and Hiragana. In Direct Input mode, what you type is not modified at all, so you can type as you do normally without it being converted into kana/kanji. The Japanese IME defaults to Direct Input at startup. So what you can do is set your default input language to Japanese IME Standard 2002 (see step 15 in the first link for where this setting is). Now everytime you start your computer, you can simply press Alt+~ (that's a tilde. The little squiggly line that's next to the 1 key on US keyboards) to switch between direct and hiragana input. 簡単でしょう?
[edit] Edited to add article for Vista.