How to disable MSN Messenger traffic in ISA server
If you're worried about losing data and confidential information or just concerned about the amount of time that your staff spend chatting on MSN messenger then have a look at this article which explains how to restrict MSN Messenger traffic through ISA server by creating a new rule.
If you need to use Instant Messaging internally at work, then you should have a look at Live Communications Server 2005 which has features suitable for use in a corporate environment, and with the Windows Messenger client should give you the ability to manage your environment effectively. And with the Istanbul client on the way, its certainly worth considering.
Go on - have a play with LCS 2005!