Have you heard of pidgin? You can use your msn account on it and others as well like yahoo. You can sign on with all these different providers at the same time in the one app. It's at:

http://www.pidgin.im/

But it doesn't have all the features of msn, ofcourse. But for basic im, it's pretty good.

What you can also consider, is the web version im, ebuddy which is similar. It's at:

http://www.ebuddy.com/

You don't have to download anything. One really annoying thing about ebuddy though is there's the add in the im window which is huge and bright and annoying...