When a computer is infected with a virus today, many IT support technicians assume the user contracted that infection as a result of visiting a pornographic web site. In fact, that represents a small percentage of the infections that are occurring with the current generation of malware. That was a much more common infection vector in years past, but not so much in 2013.
Before going into a list of ways a computer can become infected, it may be productive to state the obvious -- no rational computer user deliberately sets out to do anything that will lead to a virus infection on their computer. Some may engage in what they know is risky behavior, but will usually heed a warning that clicking on a link or downloading a file may cause their computer to become infected.