Here is what I use on all my boxes, and why I use a suite of tools as well:
Spybot - Search & Destroy, reason, it's my main Anti-Malware app and their forums for spyware removal are very good indeed. The immunize feature is great for blacklisting sites under the Restricted Sites Security Options.
AdAware PE, reason, it's my secondary Anti-Malware app.
Spyware Blaster, because they update a lot and it overlaps the blacklisting for the Restricted Sites Lists. Some things this app has that Spybot does not and vice-versa.
Spyware Guard, mostly to see if any other weird IE vectors are attempting to be exploited or not.
I used to use the Microsoft Anti-Spyware, but I think I was just duplicating the same features with AdAware and Spybot.
For free I use the AVG Anti-Virus app for my email and it supports Outlook 2000, which is more then what I need for an email client
