This was certainly a huge issue for me when I still had Windows computers; the amount of spam, viruses, malware and other unwelcome passengers in my system seemed to grow exponentially - and this, despite the fact that I had actually bought (rather than downloaded 'free' versions of) the most robust protections that were available at the time.
These security issues, - along with excellent customer service, and elegant design, plus the now well-known 'halo' effect from having purchased an iPod - were what persuaded me to switch to Apple in the first place, a move I had contemplated for around a year before I finally made the switch.
However, do you think that 'security' issues are simply related to - or, should I ask, confined to - virus attacks, or is there a wider discussion to be had on the topic, one that a suitably small sub-forum might make room for?