bbcMillions tricked by 'scareware'
The scam is difficult for police and other agencies to target
Online criminals are making millions of pounds by convincing computer users to download fake anti-virus software, internet security experts claim.
Symantec says 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 sometimes use it to get the victim's credit card details.