ok usually there has to be a part where you have to make a move for the hacker to get inot your computer by downloading a worm or something, there are cases where crackers as we like to call them can get into your system by other means than having you download a worm, trojan or some other cracking tool but that only usually happens when your computer would be a big target. Crackers that do it just to cause damage and piss people off usually just send around a link or a download that some stupid person is going to click on and download, after that click the damage is done and most people don't know how to reverse it.
With the subject of AIM, messenger and yahoo I don't see them as any kind of a threat if you know how to aviod certain risks, don't open links, or if you do copy and paste them into internet explorer or mozilla. There is no problem with AIM or messenger just the people that like to attack it.
i think you make this supject bigger than what it take,its normal guys i use Aim,icq,yahoo,msn always i have norton antiviros 2006 its really good, it make my computer in safe always.