As you may have noticed then Microsoft monthly ships you a new version of something called MSRT (Microsoft Malicious Software Removal Tool), this comes via Windows updates.
What does it do? Well its a very basic cleanup utility for certain mal/spyware, once Microsoft deems that a mal/spyware is widespread enough it is added to their MSRT and is thus cleaned from all machines that run their Windows Updates. It is NOT a malware/spyware scanner as such as it only cleans known and targeted mal/spyware and it offer no realtime protection, it runs – cleans and exits.
As mentioned all this happens behind the scenes about once a month, should you however want to do the scan again (you may be infected with mal/spyware 2 minutes after the MSRT are run, and then it will be an entire month before the scan is performed again) then you can download and run the MSRT scanner yourself (or rather a GUI version of it, the original run 100% behind the scenes).
Download it from here;
or here (I am not 100% the latter is updated regularly)
UPDATE January 26th 2009;
It’s even easier than this, see;