I use the free ClamXav.17" 2.2GHz i7 Quad-Core MacBook Pro 8G RAM 750G HD + OCZ Vertex 3 SSD Boot HD
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Sophos Antivirus for the Mac (Home Edition) is free and won an aware for best anti-malware solution.
I've been using for a while and have not had any issues with it crashing or interferring with applications and has actually blocked a number of potential websites which I'm grateful for
Which is the best firewall for Macbook Air?
Use Mac's Firewall (->System Preferences->Security->Firewall)
OSX also has IPFW. A good GUI front-end for this is: http://www.hanynet.com/noobproof/