Market share:
the mac users of the 90s were no more stupid than the mac users of today. In fact, today with the internet everywhere, users are more exposed and more likely to make security mistakes. The macs of the old days relies on floppies and local apple talk network. yet, the macs today (running a Unix based operating system) has less security issues than the macs of the 90s (single user system). a great operating system protects stupid users from themselves (to some extend). There were virus, trogans, and malware for the old macs but almost none for the new macs. Why? Because it's really really hard to write an effective virus for the new macs. Surely, if it is market share is all that matter, the new macs should at least have hundreds virus for it like there were for the old mac operating systems. But there is no known virus that can do much damage on the new macs.
User stupidity:
user's stupidity is even more a thread with shared computers. you share your Windows pc at home with your siblings. You can't know what they will do with it. you can't even assume they are smart and will do the right thing when they saw a message like scan your hard drive for virus. My cousin's little daughter was just playing flash games and barbies online and nude pictures start popping up on their computer. With Linux, you can feel free about it knowing they could only mess their account, but not the linux installation. You can even let your sibling act stupid.. be stupid.. of course, if you are stupid, that's another problem. but I do hope Linux will somewhat protect stupid users with root access from themselves.
we are all glad that Windows have made huge improvement in security. But there is much is needed.