The problem with the antivirus software is that it gives you false sense of security. Antivirus is like wearing a body armor, yeah, you can survive most of the shots, but one in the head will get you down anyway. Not using any kind of AV for 10 years now, not a single threat (yeah, doing online scans from time to time just to make sure). I just recommend caution using the computer, that's enough:
1. Use the lowest popularity browser, that is not based on something popular, like opera, very unlikely something with 1% to be the target of an attack.
1a. Or use google chrome - it is the only browser sandboxing every site you open, so even if something penetrates the browser itself, it can do no harm to the system, just close the tab and you are done.
2. Enable flash (&scripts) in sites when you actually need it - there is some bit of software for every browser that disables flash by default, leave it open for youtube and then if you want to see some in a site you trust - enable that specific one
3. Disable the autorun - infected usb flashes are so common
4. If you have some doubt in an executable, give it a hard check with not one, but 40ish antiviruses in
http://www.virustotal.com/