avast! ( http://www.avast.com/eng/download-avast-home.html)
ZoneAlarm (http://www.zonealarm.com/store/content/company/products/znalm/freeDownload.jsp).

Then, to be sure, several times a day (or at least just as I’m shutting my computers down) I will run the following, in this order:

ATF-Cleaner (http://www.atribune.org/index.php?option=com_content&task=view&id=25&Itemid=25)
CCleaner (http://www.ccleaner.com/) (NOTE: Run both “Cleaner” and “Registry” … I’ve never had a problem when I’ve “fixed” an issue found during a Registry Scan… but ALWAYS “backup changes” to be sure … it almost always finds an “Invalid Default Icon” issue with ZoneAlarm and if that is the only “problem” it’s safe to not “fix” it)

On a regular basis (at least once a week) I will run:

SpywareBlaster (http://www.javacoolsoftware.com/spywareblaster.html)
SpyBot Search & Destroy (http://www.safer-networking.org/en/mirrors/index.html) (NOTE: Don’t forget to “Immunize”)
Abexo Free Registry Cleaner (http://www.abexo.com/free-registry-cleaner.htm)

About once a month (when I’m not going to be doing anything with my computer(s) for a while) I will run:

a-squared Free 3.5 (http://www.emsisoft.com/en/software/download/)

And yes, you do have to update ALL of these regularly or they won’t help much (but everybody knows that already, right?).

HINT: To keep my desktop semi-organized, I put all the icons to these programs in one folder, which I label “Lysol”.
NOTE – ATF-Cleaner does NOT install on your computer. This very small program loads when you run it. I download all these files into their own folder INSIDE one folder (which I label “security suite”). For ATF-Cleaner, I create a shortcut which I move to my “Lysol” folder, but you could also just move the entire file.

I hope this helps!