Well, first of all, your computer, (or at least the IRC program) needs to be loading in Japanese mode. There are two ways to do this, if you already haven't done so.
1) To change the computer (running windows xp) to Japanese mode: start, control panel. In classic view: regional and language options. In category view: Date, time, language and regional options, then Regional and language options icon. Select the second tab, "languages". On the lower half check the box for "install files for East Asian Languages". On the upper half click details. Add Japanese keyboard and Japanese Input System (IME). (This is to add the ability to let you type in Japanese, you can skip it if you dont want it) Next, click the third tab next to languages, "Advanced." Select Japanese for Non-Unicode programs. Click ok, and you may or may not need the windows CD.
2) The second way to do this is just to set IRC to load in a Japanese Environment. You will need to download a new Windows addon called "AppLocale".
http://www.microsoft.com/globaldev/tools/apploc.mspxThis program will allow you to chose running IRC in Japanese mode.
After this, you need to make sure that IRC has the correct settings. If you are using mIRC, follow these instructions: Tools, Options, IRC/Messages. *uncheck* SJIS/JIS conversion and the other options. (These options are only for people who want to read Japanese, not write it)
That should allow you to write Japanese with winamp. Let me know if you have any problems ^_^
