The general consensus that I have seen on this forum is that you need an anti-virus program, have a look here for some options: http://forums.cnet.com/5208-6132_102-0.html?forumID=32&threadID=238606&messageID=2435089#2435089
In addition, I have noticed that if your behind a router that is already a step ahead, so windows firewall should be good enough, especially based on what you stated. If you look at some of the other posts in that above link section, you will also see recommendations for other programs, like on-line scans, firewalls, etc.
I have a Netgear router, which apparantly has a firewall, and Windows firewall is enabled.
I'll probably buy online in the future, but I don't have a business or any thing like that, which could be at risk.
I don't have anti virus software either. Would you say it is necessary? I don't look at porn and try my best to avoide sites which have laods of popups etc...
The closes thing to anti virus software I have is Webroot Spysweeper.