Well, I've been having unprotected sex for umpteen years, and have never gotten an STD, but that's because I'm in a monogamous relationship. I can't imagine how restricted your browsing is, if your computer has never gotten infected in that time.
Monogamous computing?
Never click on executables! Unless you know what it is...
I've currently got 3 PC's, 2 Laptops, 2 XBOX 360's, PS3, Wii, Denon 3808ci AVR, Denon clock radio and a couple of hand helds all connected to a 802.3n network. And only 1 casualty in 20 years. A firewall and encryption is all that is needed if you have a clue.