There is a particularly malicious worm (a self-replicating computer virus) currently trying to spread itself through many of the world's largest instant messaging and social networks. The worm spreads by inserting a link into an IM conversation with a person whose computer is already infected. When the link is clicked, it opens in a browser and downloads the worm. Then the process is repeated.
Because Windows Live Messenger 2009 is affected, Microsoft has temporarily turned off active hyperlinks for web addresses sent in IM conversations using Windows Live Messenger 2009. Messenger 2011 is not impacted.
If you think your computer may have already been infected by a malicious worm, download the malicious software removal tool
from the Security TechCenter on Microsoft TechNet
Additional details are available in Security alert: Security alert: Active links in Messenger 2009 temporarily turned off to prevent a malicious worm