why do u guys use free antiviruses, yes they are good, but not perfect... nod32 or kaspersky should be one everyones computer. i havent had a trojan/virus/spyware/anything since i learned what the internet was because ive been using kaspersky or nod32 on my computers. nod32 has the best heuristics ive seen - it catches homemade viruses better than piece of crap norton or mcafee.
of course this coupled with not opening random things should keep you safe.