Have to say, that common sense is what helps most. when I was working as a tech, the majority of issues seemed to be a combination of Norton and Limewire. what better way to destroy your pc than install 2 pieces of shit on it. users with norton, seemed to think, that since they paid for the yellow box every year, they could do whatever they wanted... boom.
personally, I went down the route of free for years, and never had a problem, after finally finding a paid for solution about 3 years ago that is light enough (Kaspersky IS) I have made the switch, mostly because I have it on all the family and friend computers I have to support and its easiest if I am familiar with it.
since installing KIS, none of the machines have ever had an issue. big fan.