Home computer systems are frequently insecure because they are administered by untrained, unskilled users. The rise of botnets has amplified this problem; attackers can compromise these computers, aggregate them, and use the resulting network to attack third parties. Despite a large security industry that provides software and advice, home computer users remain vulnerable. I investigate how home computer users make security-relevant decisions about their computers. I identify eight ‘folk models’ of security threats that are used by home computer users to decide what security software to use, and which security advice to follow: four different conceptualizations of ‘viruses’ and other malware, and four different conceptualizations of ‘hackers’ that break into computers. I illustrate how these models are used to justify ignoring some security advice. Finally, I describe one reason why botnets are so difficult to eliminate: they have been cleverly designed to take advantag...