Please refer to http://www.greggman.com/japan/xp-ime/xp-ime.htm for instance.

The url recommended by Jay would get you to set up your computer ready for Japanaese IME. Now, you go to wiki and find the article on Roma-ji and/or Hiragana. Use the alphabetical representation of Hiragana to type Hiragana using English keyboard. For an example, typing "a" would give you あ, press "Enter" to complete the input. As you advance, you would like to type Kanji. For an example, type "a" and "o" for あお, press the space bar once. The kanji, 青(blue) will show up, if this is the Kanji you wanted, then plress "Enter" to complete. If this was not the Kanji you wanted to type, then press the down arrow key to view the toggle down list of Kanji's. 　

If you are a Windows user, seach multi language pack on Microsoft website. There should be a patch for Japanese input on English operating system. Once you install it, then input with "Romaji" mode, for example if you want to input "ありがとう" type in "arigatou". Then to change it to Kanji, usual manner is to use space key. Enjoy!