Generally when you have a virus, you'll notice it. Most viruses are ad aware type, or spaming. They try to get you to buy their software, open IE all on its own, basically anoying.
There are other viruses that attempt to use your computer for illegal purposes... these can be dangerous but stopped with proper fire walling.
The only real virus i guess you could fear is a spying type of virus, which I guess i really dont know if I've had one. I regularly check my start up processes, in addition to everything that's running in the background.
Staying off porn sites and other silly sites is basically a shure fire way to not get a virus, I do torrent things, but I make sure and read up as much as possible, and I test them on a different OS before adding them to my current one. IDK, i just find it pretty obvious when or if there's a virus... I can confidently say " I havent had a virus in years..."