I've been using AVG Free anti-virus, but I no longer recommend it. 

The link scanner is a good idea in theory, but a bad one in practice, and will slow down the computer.  I suspect on an incredibly fast business Internet connection it would be OK but not for a UK home user.  And what's more when you turn it off, the software permanently indicates that it has an error.  BAD BAD BAD user interface.

Then there's the killer issue - it stops printing working on IE7!  I can't be bothered investigating why.  I used to like it because it was so simple and never caused me any problems, now its caused me 2 significant problems.

My current recommendation is to not use anti-virus, if you have an all up to date version of Windows with windows defender installed, and the firewall up etc with a firewalled router.

Link to my Flickr photo set