I find irssi to be an excellent Linux client, and if I had to use Windows I'd probably use it there, too.
Have you ever actually used IRSSI on Windows? Maybe the Linux version is better than the Windows one. The build I got for Windows (the latest as of a few weeks ago) has some very bad display problems that make it, at times, very confusing and hard to use. I'm not sure if these are bugs, or just very bad design choices.
Besides, it's lacking everything that makes Windows, well... Windows. It's a total throw-back to the bad-old days of DOS. Yes, command-line interfaces can do some stuff really well. (Scripting, batch files, etc.) But making a user-friendly program is not one of them.
