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You don't need any antiviruses for OS X because OS X has got its own security systems. See > http://www.reedcorner.net/mmg
However, if you want to install an antivirus, we only recommend ClamXav and Sophos. Other antiviruses just can make your computer slower and, in some cases, damage OS X.
Note that this only applies to OS X. If you install Windows on your MacBook Pro, you have to install an antivirus in Windows to keep Windows safe from viruses. In this case, I recommend Microsoft Security Essentials
WARNING: "Avast" is useless garbage that will cause you nothing but problems.