Mac OS X Leopard is UNIX. I am, by profession, a UNIX admin. I know how to secure a UNIX server, and in the seven years at my current job I have yet to deal with a virus or other security breach. I have, however, had to deal with Windows admins, and perhaps it's simply anecdotal, but in my experience they generally do not know how to secure their servers. That is because, while UNIX makes rational sense to me and other UNIX lovers, and has in fact been refined over the past 30 years, Windows seems to baffle even Windows professionals who must get re-certified every time Microsoft updates their OS and changes everything around.
You don't have to surf porn or visit websites or even do anything at all to be vulnerable to attack when you run Windows. Here's a funny story. At work I use Linux. Linux does everything I need to do except one thing, which is to access the company's trouble ticket system called Remedy. So I used a product called VMWare to install Windows XP in a virtual machine running on my Linux laptop. This virtual Windows was never used for anything other than Remedy. No websites visited. No nothing. But it caught viruses. Not the Linux host machine, but the virtual Windows machine.
Viruses are just part of life when you run Windows. It's not because of Windows' market share, or questionable websites visited. It's just that a Windows machine, on a network -- even a virtual Windows machine on a "protected" corporate network -- is inherently insecure.
When I was a kid and became interested in computers, one of the things they said about computers was that they don't make mistakes. Nowadays, people just assume computers "mess up." Computers are finicky, they crash and lose your files, and everyone just assumes it's par for the course. But it's not. It's just Microsoft. People should be fired for choosing Microsoft.
Someone here said that if you keep on top of it and update your Windows computer with current anti-virus software then you shouldn't have to worry. Personally, I prefer not having to do anything special in order not to have to worry, and instead like to spend my computer time on my own terms.
That's why I bought a Mac.