Here's Bob Rankin (Read more: http://askbobrankin.com/why_did_my_computer_get_a_virus.html#ixzz1UdszyfAf") talking about how our computers get infected:
"There are many ways a computer can become infected by a virus, Trojan, spyware, or other malware. Many of them depend on your cooperation, or at least your inattention. Below are some of the most common ways that your computer can get infected, and suggestions for preventing it.
Clicking without questioning is one of the best ways to get a virus. Whether browsing the Web or installing new software, many people just "follow the prompts" given to them by a Web site or installation program. Malware distributors take advantage of this mindless behavior, prompting users to ploys that seem crude but actually work quite often.
A favorite trick is to pre-check "permission" buttons in installation programs, implying that the "default" thing to do is accept whatever malware-laden toolbar or add-on program that is offered.
Scare tactics are often used to induce hasty clicks. A pop-up window may scream, "YOUR PC IS INFECTED!" and urge you to click for a cure. Often the "cure" is really the disease, which did not exist on install" is another con that malware pushers use. ("Doohickey" is a made-up name, not a real plug-in.)"
Click on the read more link to see a great chart for protecting yourself on the internet!