Microsoft on Friday forced its millions of MSN Messenger users to download a new version of the software to plug a vulnerability discovered earlier this week. The mandatory upgrade began early Friday morning after a security company posted a how-to guide describing how the vulnerability can attack computers. MSN Messenger users were then greeted with a notice to upgrade before they could open their instant messaging clients.

The moves come after Microsoft on Tuesday released a series of software patches to plug a series of "critical" weaknesses in a handful of products including Office XP, Internet Explorer 6, Windows Media Player and MSN Messenger. Microsoft was planning on asking Messenger users to upgrade over a longer period of time, but once the security posting went public the company decided to upgrade all users at once, a company representative said.
Source : http://news.zdnet.com/2110-9588_22-5573252.html

That a good move from Microsoft part! They react quick to the thread I post here