nAIM is availible for xDSL. Gaim is availible for Xebian and Gentoox.
Other than that, I don't see much alternatives. The reason people are against making an AIM program is because the Xbox dashboards can only load one program at a time. Imagine a simple AIM, with not being able to view internet, listen to music (from the Xbox), or play any games.
Just get one of the Linux distros... I recommend xDSL because it is not a memory hog. Even if you have no idea how Linux works, xDSL is easy enough to navigate in.