I tried using anti-virus software for a while, back when we got free copies of Norton at NC State. But it seemed to have too much of a performance hit on my system, so I disabled the real-time protection and just used it to scan files I was unsure of. Now I'm not using any, and it's never been a problem. Nearly all viruses exploiting security holes in Windows only work if you are plugged directly into the internet-- a simple Linksys router makes you pretty safe. Plus I don't use Outlook or Outlook Express, and I never run software from untrusted sources (or anything from an e-mail that I wasn't expecting). And Microsoft has done a pretty good job of plugging those holes (IIRC that was a big reason why Vista was delayed a year- they had to switch a bunch of resources to combing for security holes). And if I were to get a virus of some type, I am quite capable of reinstalling my OS and I backup all my important data fairly often.

Yeah, I agree. If you are behind a NAT router and you surf the web safely, you should not have to worry about having AV running. I don't have AV installed on my laptop at home. Although, it is probably a good idea to download AVG every once and a while and scan the system. Examples of surfing the web safely include: Going to only trusted websites and possibly running with scripting turned off.

I think a good rule of thumb is: safe browsing = not Internet Explorer :)