I run two desktop PCs and one laptop daily. None of them have any anti-virus/anti-spyware software, and I am constantly downloading files and going to sites I shouldn't. I've never had any problems with viruses. Why? I know wtf I'm doing.
To the dude that said "my sister goes to MIT." First, you sound ******ed saying that MIT bull****. "OMG she went to MIT she must be the smartest ever!!!" Second, I know lots of people that have prestigious comp sci. degrees that I can code circles around. And I have an economics degree.
Macs are designed to be streamlined. The software and hardware is built by the same company, so clearly they're going to be more user friendly and have less problems. Windows OTOH is designed to work with thousands of different configurations, and it's virtually impossible that it will work perfectly on all them. There aren't viruses for Macs because a very small percentage of people use them. Why write a virus that will barely effect anyone? Personally, I prefer PCs because I can customize/upgrade them cheaply and with little effort. Macs are great for people who just don't want to bother with that. There's no winner in this argument, it's all about what works best for the individual.