Is your anti-virus program good enough? It is probably written by a large, reputable company, and updated frequently. Its probably good at detecting viruses and keeping your computer locked down. But it has limits. As this AV-Comparatives report shows, not all anti-virus programs are made equal, and NONE of them can stop every intrusion: http://www.av-comparatives.org/images/stories/test/dyn/wpdt2011_1_en.pdf
So its not good enough to just get an anti-virus program and call it a day. You need to be secured as well. One of the best ways of doing this is to lie to yourself. Tell yourself that you don’t have an anti-virus program installed. No anti-virus program is perfect, so why act like yours is? Would you click on that link if you weren’t protected? How about run that program that downloads more ram for you? It isn’t good enough to rely on the computer to protect you. It’s your job to protect it.