Treat everything you do online as if you have zero privacy.
That's exactly what many everyday users get wrong. They think surfing the net is absolutely anonymous and secure -- clicking on random links and installing random software has no consequences. Later they can't understand why some script kiddie has compromised their entire system. Other than that, I agree that getting the most secure online experience requires turning off the computer and locking it inside a safe, but that's hardly useful. Your can live quite privacy-preserving online life, if you are careful.
Too many people are thinking of security instead of opportunity. They seem more afraid of life than death. -- James F. Byrnes