MSE was good, however it didn't flag things until they made a rogue move and attacked a system file.
Norton.... Expensive, bogs the system down and has tons of false positives. My 'The Sims 2' constantly crashed because it would flag the texture engine.
But none the less, I love avast, it's light weight and live scanning, with optional on-demand scanning. Plus if I really wanted I could disable the notifications for updates or mute it.
"Norton.... Expensive, bogs the system down and has tons of false positives." - This is not actually true, if you haven't configured norton it will detect even files that have not been scanned before to their database or files that haven't been seen much by other users. And this may have been more true about it bogging down the system in pre 2009 versions, but as of 2010 and later, it's not that bad. It does well though, so if you want the security, you can't complain, because as you say, and unlike MSE it will analyze much more for you. MSE is nice for a lightweight program however, but if others did as much as Norton does, they would be just the same.
Norton by default also is configured to trigger a notification for keygens, whether they are malicious or not, as well as hacking tools. So although these are not good to be using (because of the illegal activity that usually goes along with them), obviously they will be identified as false positives (if by definition that means detection even for clean/non-malicious files), but ONLY until configured not to be detected as such. You also need to configure the networking area for allow/deny options, as this will affect the detections too at real time. It's configured by default to be very strict though because I can say with confidence that 80% of computer users using Norton probably won't have a clue about some of the options in there. (Thus, I would consider it's strictness by default a good thing, because less experienced computer users probably would be better off with more strict settings.)
Too many people as I mentioned though, just don't configure Norton, because they are used to the lack of configurability given by other similar AV programs (?... Or just perhaps believe that everything should be, by default, set the way they want it to be configured?). Norton has much more in regards to settings than any other AV I have seen though. BitDefender does other very nice things that Norton doesn't, but it lacks the configurability that Norton has, and I think Norton's firewall for the networking settings is better.
ESET is recognized as well, but it's the top contender for false positives, way above anything else. But if you are after security, then a few detections that may contain a handful of false positives and "true" positives, instead of not detecting something malicious is better than nothing I suppose. You can configure Norton to detect nothing however (without turning the AV off).