Apparently, Norton is no longer slow and MSE no longer offers effective protection: http://www.av-test.org/en/tests/home-user/marapr-2012/
You can also check out av-comparatives.org for further antivirus tests but it doesn't test Norton nor MSE.
I am not going to make any recommendations. Just regularly read the latest reports from av-test.org and av-comparatives.org and use the product that consistently produces good protection.
I have often benefited from recommendations for Things I Didn't Know I Wanted.
Given that Less Wrong is a community of unusually intelligent, critical, and self-improvement-focused people, I suspect we can generate a pretty helpful thread of product recommendations — perhaps even a monthly thread of product recommendations.
Rules: