The troubling findings underscore the second annual AOL/National Cyber Security Alliance (NCSA) Online Safety Study just released.
The study shows more than half (56 percent) of the participants either had no antivirus protection or had not updated it within the last week, almost half (44 percent)) did not have a properly configured firewall and nearly four in 10 (38 percent) lacked spyware protection.
The study also shows that unprotected consumers are making themselves prime targets for online scams: three-quarters (74 percent) of respondents use their computers for sensitive transactions such as banking, stock trading or reviewing personal medical information.
In addition, more than two-thirds (68 percent) keep sensitive information on their home computers such as personal correspondence, resumes or professional records or health or financial information.
"There is a major perception gap: Even though most consumers think they are protected, this study shows the opposite," Ron Teixeira, the NCSA's executive director, said in a statement. "Far too many people still lack the three fundamental protections they need to stay safe online -- current anti-virus software, spyware protection and a secure firewall."
The AOL/NCSA study is the largest study of its kind, sending technical experts into hundreds of typical homes to examine PCs for known security risks and threats.
One of the key consumer misperceptions, according to the study, involves phishing attacks, with scam e-mails aimed at identity theft hitting one in four U.S. computers every month. The study shows more than two-thirds of consumers (72 percent) thought the phishing e-mails were from legitimate companies.