As mentioned by Dana:
A new version of MBSA was released yesterday to allow users on XPSP2 to take advantage of the tool.
In case you didn't know, 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.
I downloaded and ran this tool this morning and everything checked out on my new Windows XP build (with SP2). After my problems last week, I spent some time offline this past weekend and paved/rebuilt my Windows XP partition on my laptop. I made sure the required virus protection software, and other tools, plus SP2 were on the machine BEFORE connecting to the internet. Tools like MBSA verify that the right service packs and settings are in place as well.