I haven't run an anti virus in about 3 years -- I used to have Symantec but it takes up a significant amount of memory / processing time and the hooks into the OS are ridiculous and good luck uninstalling, if you do one step incorrectly you're in for a definite reformat. AVG has similarly deep hooks, but it seems removing AVG is a bit easier.

I just don't download sketchy shit or go to sketchy websites -- all my questionably-legal downloads are done through torrents that have high ratings (I'm talking only certified uploaders on tpb) and if I have any question I use an online virus scan and upload the file.

I have a pretty legit firewall on my router and to be honest, most of the time my boxes behind the router have their firewall off (I should probably turn them on, though, I'm just lazy).

I haven't gotten a virus since.

Also, I reformat like once every 3-6 months anyway, backing stuff up to external hard-drives / network shares / etc.