AVG Free is fine enough as it the
AVAST free version. They do get a bit naggy about updating when they try to get you to buy in but they do a pretty decent job - AVAST would be my choice of the two at the moment.
Microsoft Security Essentials is also free and is what I am currently using at home for testing although I'll probably switch to
Sophos as I get that free.
Norton isn't soo bad these days but they trashed their reputation with IT people and we haven't forgiven them for their bloated apps.
If I had to buy one I'd probably go for
Kaspersky as they rate highly and you get to cover three PCs in the licence. It does very well in
real world protection - There are many other decent AV progs like
Trend Micro,
BitDefender and
F-SecureOf course AV progs now have morphed into different beasts offering all sorts of extras like Child Protection, IM/Chat protection, email scanning etc most of which are superfluous to many users so you do need to look at what you are buying.