Most AV software today are very competitive, even the free ones, so you should not worry about viruses if you have one of the top recommended AVs. Having additional antimalware programs makes sense, as some others have pointed out, as long as they do not slow down your computer. i currently have a subscription to GData Internet Security, which contains two engines preconfigured...Avast and BitDefender....and also has a good firewall.
One antimalware that I would recommend, if you do not mind paying for it, is Hitman Pro 3.5. You can download a 30-day free scanning and removing trial version of it here to see if you like it:
http://www.surfright.nl/en/downloads/
Hitman Pro contains the following scanning engines: Dr. Web, IKARUS, Prevx, G Data and Emsisoft, which are configured to be compatible with each other and with whatever antivirus program you have installed.
Hitman Pro v3.5 is not a substitute for a good antivirus program, however, and there is always a chance of false positive detections. You should check any "malware" detected to make sure it really is malware before deleting it. Tracking cookies detected can safely be deleted.
Ricochet_16