Can I pitch in a little here. Although I'm a Mac user and always have been, I found that after messing around with raspberry pi an Openswan vpn and all the other guff that it leads to, that a good starter with anti virus is to lock down all of the unnecessary and redundant ports on your router.
The malware and virus bots that trawl the internet looking for computers to infect go for the weakest point in the first interface to the unprotected internet/DMZ (DMZ= De-Militarised Zone-geek speak for the unprotected internet, where all the information and horrid crap is), which is your router.
As standard when you purchase a router from your service provider it comes with a load of ports open, such as the ones used in gaming, when you want to share a game over the internet (viruses and malware love these ports). If you don't use these ports then I suggest that you close them down in your router. A bit like locking your windows and doors at home.
You really only need to have the ports that you use open and none other.
For example I have 8 that I use open. You probably have 15 or so open that you don't know about.
Mine are;
Web- https
mail- smtp, pop3, mail-ssl
remote access- Secure Shell Server
VPN-IPSEC (if you run one from home or work)
Website-dns-udp (if you host your own)
Server port.
This will provide a robust and free first line of defence that is administered by you and not anyone else. Also change the username and password to your router from the standard which may be root, root or admin, admin. How easy is that to guess, doh!
The next line of defence should be keeping all of your software up to date. As mentioned before by other posters.
Then consider paying for 3rd party software as a last resort if you still don't feel comfortable.
You only need to do a few simple steps to provide security for yourself. I do have free Sophos antivirus and also Trusteer which is provided by my bank foc. My home/office firewall is provided by my locked down router and base station wifi and they are all protected by difficult to guess passwords and usernames so no one can stand in the street and access my wifi or router.
Hope that helps a bit and is not too confusing to understand.