anti-virus
It is definitely all about training and knowledge of malware, I don't have any anti-virus on any of my PCs, but how many users are smart enough to identify and stay clear of the bad stuff? Experience is the best defense. No anti-virus is perfect, some don't even come close, and most cannot clean a PC completely after infected. But we all know this already. Knowing this I would not pay one penny for anti-virus, there are plenty of free ones that work as good as you can expect. The best anti-malware software I have ever used is free...combofix.