Its true the mac has very few virii, but Apple does have many security holes in the base Mach system they refuse to repair. I was reading a security article on slashdot a few weeks back where bugs are submitted but Apple will not fix. I guess Apple doesn't need to worry about security, since few use the Mac OS, malware writers have no need to code (exploit) the OS. You don't go fishing in a desert.
the best anti-virus is not getting one in the first place. stay out of brothels and porn sites IOW, HAVE A DEDICATED SYS FOR TRADING, and don't steer it anywhere but at your broker
the symantec antivirus is one of mine, it always have the update package for all people so that they can scan more efficient. can try because it's free