Shoutbox

I fail to see how this has anything to do with Plus!. Reinstalling Plus! would not have fixed an incorrect username/password notification, it must be a coincidence.

As to how they did this, it sounds to me like your MSN simply became overloaded with emoticons, presumably the "attacker" was unaffected because (s)he had them disabled. I don't know why you were unable to sign back in though..