MSN Messenger is no more!
You will all have to upgrade to Windows Live Messenger.
It's still the same old MSN, just with a new name. So you can't really call it "MSN" anymore.
It's "WLM" or "Messenger" now.
Download it free here: http://get.live.com/messenger/overview
If you have MSN Plus! 3, please uninstall it before updating to the new Messenger, as the new Messenger will only work with the new MSN Plus! Live 4.