You said PC, so here's how to do it on Vista:
http://windowshelp.microsoft.com/Window ... 91033.mspx
Here's XP:
http://www.greggman.com/japan/xp-ime/xp-ime.htm
Here's how to do it for a mac:
http://redcocoon.org/cab/mysoft.html
When you type, you basically just type it in romaji and the hiragana will appear. You'll then be able to select the kanji or katakana by pressing the space bar and then scrolling up or down with the arrows. It takes a minute to get the hang of, but it's not hard. One thing that took me a while to figure out was how to type small hiragana and katakana characters like ぁand ァ. To do this, type l and then the character you want, so l + u = ぅ but u = う.
