I'm using MSE here.
I used to be really hot on virus protection. Spent countless years investing in paid versions - Norton, AVG, Kaspersky etc...
Right now, I'm in the mindset of just watching where you go online. It's really that simple - However, as someone above mentioned, sometimes things slip through.
If experience is anything to go by, adverts, be it on the page or in a pop-up, are generally the main cause of involuntary virus-downloading.
I'd bet if you went to even a mainstream porn site without an ad blocker you'd get yourself some digital aids pretty darn quickly. Pop ups that appear when clicking on something, or anywhere on the page. Banner ads - etc... An ad blocker is useful in more ways than just removing annoyance.
Also worth mentioning is to set your browser to ask before downloading - I've come across a few instant-pop ups that immediately download something without consent in the past.
After that, it's just a case of watching what you download really. Not the hardest thing in the world to do.