US computers users know more about Janet Jackson's breasts than about security software on their own PC.
PC users remember when Ms Jackson was exposed
A survey found that 90% of those asked could remember when Ms Jackson suffered her "wardrobe malfunction".
But only 60% recalled when they last updated anti-virus, firewall and operating system software on their PC.
The survey, by a US security group, found people were getting more worried about computer security but some vastly under-estimated how at risk they were.
Commissioned by the National Cyber Security Alliance (NCSA), the survey found that 30% of people believed they had more chance of getting struck by lightning, being audited by the tax man or winning the lottery than they did of falling victim to a computer security problem.
PC users aged under 25 were even more sure.
Americans have little chance of being struck by lightning
40% thought they would get hit by lightning, or suffer one of the other events, before being caught out by a computer security breach.
In fact, said the NCSA, people are far more likely to be struck by a hack attack than atmospheric discharge.
According to the US National Weather Service, Americans have a 0.0000102% chance of being hit by lightning.
By contrast the chances of falling victim to a computer virus, phishing attack, malicious hack attempt or other cyber security dangers are currently running at 70%, according to statistics gathered for the E-Crime Watch Survey.
"Cyber-security should become second nature, just like brushing our teeth," said Ken Watson, chairman of the NCSA.
The majority of people did pick out computer security as the biggest risk of the four choices they were given.
The NCSA has declared October to be National Cyber Security Awareness month in the US and is running a series of events to warn home users, small firms, educators and parents about the true scale of the threats they face.
Mr Watson said 91% of PCs were infected with spyware programs that spy on browsing habits and report what they see to spammers and others.
The NCSA is planning an in-depth follow-up study to its survey in late October that will conduct technical examinations of home PCs to see how well protected they are and what risks they face.