For what it's worth, AV-Test has evaluated free antivirus software as well (even in this test), and it generally works almost as well as its paid counterparts. You don't have to shell out a lot of money to keep your computer safe. No matter how careful you are with your computer, I say you've got nothing to lose by downloading a free program, keeping it updated, and running a scan every month or so. The law of averages suggests you're going to pick up something nasty sooner or later, and pretending that your computer is immune to malware will not make it go away.