The March 2009 release contains 3 new bulletins, 1 of which has a maximum severity of "Critical". The first of the two bulletins rated as 'important' addresses a single vulnerability in the operation of the Secure Channel component in Windows 2000, XP, Vista and both Server versions. If targeted, that flaw could allow for spoofing attacks.
The third bulletin contains fixes for four different flaws in the DNS and WINS components which could also allow an attacker to spoof and redirect network traffic. The update only applies to Server 2003 and 2008, other versions of Windows are not affected.
More info: Microsoft Technet