I use......
AVG Free
Kerio PFW Free
http://www.kerio.com/dwn/kpf2-en-win.exe (Grab this anyway, I hear it will likely disappear in the near future.)
Adaware Free
Spybot Free
MS Giant Antispy Free
Spyware Blaster Free
Highjack this! Free
Highjack this online log checker Free
http://www.hijackthis.de/
Pest Patrol not Free.
These are good but unfortunately not free.
Counter spy, Ewido,
Here's a good forum where you can get up to speed on Security matters.
http://www.wilderssecurity.com/index.php
HTH
Mick.