The scam is difficult for police and other agencies to targetOnline criminals are making millions of pounds by convincing computer users to download fake anti-virus software, internet security experts have claimed.
Symantec say more than 40 million people have fallen victim to the "scareware" scam in the past 12 months.
The download is usually harmful and criminals can get the victim's credit card details into the bargain.
The firm has identified 250 versions of scareware, and criminals are thought to earn over ?750,000 a year.
Franchised out
BBC crime reporter Ben Ando said the prevalence of scareware suggests cyber criminals are not bothering with viruses anymore.
During the scam, warnings pop up on the computer screen telling users their computer has a virus, and directing them to a website where they can download protection for up to ?60.
Symantec says some 43 million people have fallen for the scam in the last year.
It is becoming so popular that the rogue software has even been franchised out.
Because the individual amounts of money stolen are so small, the scam is hard for police and other agencies to target.
