Another IM worm, this one affecting AOL IM. News.com: “AIM Worm Plays Nasty New Trick”. From the list in the article; it loads crapware from the usual suspects, plus a 'rootkit' that will grant unwanted external access to your system. Check out “Don't be Tricked: Beware of Zombies” for the latest news.
Avoid clicking on links in ANY instant messenger client, even if from friends.