Egad. Well you may not do it but someone else in your household might. I had this problem a lot last year while living with my ex-bf and his cousin (the latter of whom had a very serious porn addiction). Our computer was constantly dying from all of the crap that accumulated on it.
   
If you're the only person in your household, it was downloaded without your knowledge from other sites. It's not so much about what you look for online as where you go to look it up. You can download all kinds of crap without knowing it, just from the sites that you visit. Best thing to do is clear your hard drive and start anew. It's best to do that on a regular basis anyway, to keep things from building up. Think of it as a clarifying wash for your computer.