Hi Keith,
To enter Kanji via the keyboard, you'll need an appropriate IME (Input method editor). I'm not so familiar with IMEs for Japanese, but I use a program called Pinyin Jiajia to enter Chinese. You type the romanised equivalent and can then choose from a number of characters which have that pronunciation. There is a Microsoft version as well, and I'm sure Microsoft must provide a Japanese IME for windows, so it's worth having a search.
With these types of IME you do need to know the standard romanised forms. There are other methods for entering characters based on radical and stroke count, but I'm not so familiar with these.
The best option I've found if you don't know the pronunciation is to use a service where you can hand draw the character and it will bring up a list of possible options. There are smartphone apps for entering characters this way. The online Chinese dictionary
http://www.nciku.com/ has this function too, and it's often where I turn to first when looking up an unfamiliar character. Nciku do say they'll launch a Japanese dictionary in future, but it's not running yet. There may be online Japanese resources which offer this service though. Failing that, you can always find the character using Nciku, then copy & paste to look up the Japanese pronunciation.
Steven
