Blurb from http://www.microsoft.com/mbsa:

New version, MBSA 1.2.1, needed for Windows XP SP2 compatibility: Users of Windows XP Service Pack 2 will need to update their MBSA to version 1.2.1 for compatibility with SP2 security improvements. Windows XP SP2 users who are running MBSA 1.2 will be automatically notified when they run the tool from the Start menu with an Internet connection.

MBSA is the free, best practices vulnerability assessment tool for the Microsoft platform. It is a tool designed for the IT Professional that helps with the assessment phase of an overall security management strategy. MBSA Version 1.2.1 includes a graphical and command line interface that can perform local or remote scans of Windows systems.