Seeing as how only The Ultimate and Enterprise editions of Windows 7 are MUI (Multilingual User Interface), you don't need to worry about language packs having a negative effect on your computer use. In fact, the only way I know to activate a second language in other editions of Windows is through the non-unicode options (which as Narmy mentions will have side-effects, as it'll effect all applicable applications, I have a few applications in Japanese because of this, thankfully I know how to navigate UIs and websites based on UI design that's generally good enough to help me through

).
If you need to write out the characters, you need an IME. Windows since XP includes a basic IME (which will be usable after switching your nonunicode settings, don't think there's a way to force it to appear before then). There's also third-party ones like http://www.google.com/intl/ja/ime/, which is incredibly useful for when typing popular anime character names
