Security: Windoze, linux and mac
Though I have never used a mac, I switched from windoze to linux last year, primarily due to security concerns, and secondarily because I was beginning to get afraid of Microsoft Economics. I have some experience with linux desktop security as an average user, and I'd like to share it here, hoping that the same principles apply to macs too (as it's based on freeBSD)
Firewall: Linux has built-in firewall. One just has to configure it using iptables or a gui-enabled program like firestarter. The firewall is pretty transparent to the user and shows all the connections to and from the computer, and the rejected packets/connections in red color. I kept the linux desktop on for almost 24 hours a day, and it never got owned/hacked despite reckless web surfing and hosting websites from my desktop using Apache. Absolutely flawless security in Linux!!
Antivirus: Clamav (=clamaXav for macs) was included with my Linux distro (Ubuntu dapper drake). It was just a scanner, not real-time virus monitor. I never had any viruses on my linux partition despite downloading tons of stuff. I guess it may be because the stuff was largely open source softwares, and the community does an impeccable job at reading the source code. I got a virus from a friend on request (he got his PC killed), and clamav was able to clean that virus, which earlier had evaded norton antivirus on my friend's PC.
In about 10 years, I got viruses about a dozen times on windoze. In about one year of Linux use, I got none. This might have happend because few/no wild virii exist for linux (same with OS X).
In a nutshell, I am fully confident about the security of macs, but will have clamaXav antivirus for scanning my emails etc. lest it should infest my friends who use windoze. I'll also tinker around with mac firewall to see the performance, despite being behind a firewalled router.
Thanks for reading.