based on my experience of getting viruses (or nastier worms) since i got my hands on computers, softwares are only a complement and added security.. there are really only three practical ways of me getting a viruses / worm and here's what i would do to prevent them or to the least minimze my risks
1. script in a site
which you can prevent it with adding noscript plugin on firefox.. this disable like all interactive websites but once you turn it off, its your call and your risk, so only make exception on site you trust and browse carefully... always for example checks for legitimate URLs that make sense.. i wouldn't easily trust a URL that reads "free-sexy-movie-download.info" and the link doesn't lead to a forest of sublinks of /words/words/words/words/words/stuff.html
2. physical removable medias
USB sticks, memory cards, PSPs, you name it! this is which responsible for like more than 7 out of 10 cases i got comin from this.. the way to prevent this is simple, you would want to consider sharing USB sticsk are like sharing condoms
only put USB sticks you use personally this is your best chance to avoid viruses that spread from phsyical media.. upload/email files that are from media you got from others using other computer and download them into your personal computer is the safest way to transfer files from risky physical media (unless the file you wanted is infected in the first hand) and of course like condom, try not to share this on public computers or those that are accessible / used by alot of ppl
3. files you download
that said nuff.. only download from sources you trust, if you are dl ing from something like torrent.. see comments and read what ppl say, if alot of them saying "WTF DUDE! YOU GOT ME INFECTED" and the next 20 ppl says "SH*T" then you know what to do.. not being the first is a good thing for me
i'd say i did this as a habit now and when you do it properly you shouldn't even need a firewall or antivirus (which i don't reccomend anyway since another extra layer of defence is always a good thing) this however would very very greatly prevent the anti virus from kicking into action in the first place which is much better than curing your computer