Try Trillian at www.trillian.cc -- actually they just released a new version which has seriously improved impressiveness. In some ways it's nicer than Adium.
Off-topic, but I hope you're also using OpenOffice.org (http://www.openoffice.org/). You may have to use Windows for whatever reason, but you don't have to contribute more money to such an unethical organization!
Anyway, on topic, back when I was a PC user, I used Trillian. I enjoyed it for it's skinnable interface and it's easy-to-use. Never tried GAIM on Windows (used it for a few minutes on Linux, wasn't that impressed, but then again I only used it for a few minutes).
Doesn't Adium use libraries that are furnished by GAIM? I've never tried it out. I should. I didn't really like Trillian until this version that came out in the last two weeks. Now I think it's very impressive -- much prettier than it used to be. And also it now has multi-client AV chat support, although I haven't really used this part.
Hey, did anyone else notice that in the latest Trillian, words in the chat window which have entries in Wikipedia are automatically highlighted, and then when you mouse over them, you instantly get the entire Wikipedia entry as a tooltip? It's mostly geektoy, and it gives funny definitions, but it's really fun nonetheless. For instance, I did not know that 18th century European gardens had a lowered border called a "haha."