I've been using Comodo on one of my machines - works like a bomb. Not sure if its multi core though? But you should hardly notice any performance effects anyway.
If it's for the PC in your sig just get Microsoft Security Essentials for antivirus (it's free), and use standard Windows Firewall for firewall.
We use MSE in our work environment of 50+ computers and so far have had nothing but good experiences with it, and the built-in Firewall in Windows 7 is much improved over the version in XP.