A quick look through Google didn't find my anything about this, but if Yahoo Messenger allows scripts to be downloaded, then I would imagine this is possible.
Probably not by an amateur hacker, although if the instructions were simple, then why not.
I see you have downloaded and installed a new version of YM, but I would suggest you change any password you use with it as well.
Is it possible for someone (amateur hacker?) to shut down my Yahoo Messenger? Perhaps this is just a fluke of Yahoo but this happened soon after a questionable person tried to chat and I did not want to. Yahoo Messenger was working fine until then.
Just wondering... I have downloaded the newest version now and so far it's OK.