My personal experience:
I have used AVG for a few years now and it has caught a number of nasties on my kids' PC and one or two on mine. I installed it on mum's PC a year or so ago and she hasn't been infected since.
A colleague had Nortons, and it was causing problems as far as using system resources and they were getting continual notifications from Xtra and friends that they were sending infected files by email. Nortons didn't pick up anything, due largely to them not updating it regularly I guess, mainly due to costs involved in subscribing.
I used an uninstall program to thoroughly get rid of Nortons and installed AVG. AVG identified and quaranteened several viruses.
AVG updates the definition files regularly. Installing these now is very fast. You can schedule this to occur when you find it convenient. Mine starts at 12.30am each morning if the database is more than 3 days old.
I received a copy of Nortons with a new motherboard, and tried it out, but, to be honest it was too obtrusive for me, and slowed things down too much.
Maybe for those who don't mind many apps running at once and not clued up on what to look for or download and accept every file out there Nortons is a good choice, as long as they keep it updated and don't feel jaded at paying for it once their initial period of use is up. This, I feel is Norton's biggest downfall.
J