Where I work (18,000 PCs) we currently use PestPatrol for the majority of our clean up. In our testing, we found that the Microsoft product (now call Defender) was the best product currently available. Note that the 'update' feature of Defender is semi-functional, and that to get the updates for it you're best off going through the Microsoft Updates page.
It should be noted that Microsoft is going to soon begin testing an all-in-one product that will cover Mal-ware, Spy-ware, Virus-protection, etc... Beta 2 is due this week or next with large scale Beta release near the end of hte year.
Norton and its ilk are...well... lets keep this friendly.
-Lego