I am not sure what type of windows you are using, but basically (if it not win7) you will need to install eastern alphabets from the windows CD. You will find language options under STAR menu >>> control panel >> Language options.
After the install, you will have a language bar at the bottom part of you screen. You can then change between fonts. While in Japanese font mode, you type phoneticall using english keyboard (for ex. ame for 雨 (rain) and use space bar to select either one of the syllabaries (kanas) or kanji, then press enter for input.
also look here:
Japanese Font