I personally just use MSE and Windows Firewall (Or Windows Defender on Windows 8 ) and nothing else. I personally feel that the whole belief that you need to practically wrap your computer in bubblewrap to prevent it getting any viruses is nonsense. As long as you are sensible you should never have any problems. And as for software that supposedly optimises/cleans your PC/registry - I've never touched any of them or even manually tried to clean up my PCs, and they still run perfectly!
Common sense is the best antivirus!
I agree, most of my customers who have a virus are the ones who fall prey to all the scams and install malicious software. They generally have a ton of crap running at startup and have
at least one worthless toolbar in internet explorer
Or people who use P2P and download an exe file they think is a movie or music.
When trying to decide which anti virus software is better, it's a good idea to do your research on independent websites. Some sites are likely to base their decisions on how much money a company gives them. Norton and McAfee appear to be juggernauts in this area. Their advertising may be the best, but not their software.
Personally I recommend Kaspersky as it's light weight and has been scoring great in tests for quite some time now. It will often catch things other anti-virus software didn't.
For me I generally just use the free version of malwarebytes and just do scans every so often. I have enough common sense to know what not to install and click on. Before I install something I download I usually just check the file at virustotal.com which scans the file using several different virus scanners.
So yea, common sense has worked great for me so far.