Fake antivirus software that tricks web users into believing their PC is infected accounts for 15 percent of all malicious software found online, says Google.

A new report from the search engine giant also reports that fake antivirus software makes up 50 percent of all malware delivered by advertising.

"Fake Antivirus is a quickly-growing attack trend," Google said.

Fake antivirus software convinces PC users their machine is crawling with malware, and that they should pay a one-off sum to install software that will remove the threat.

It can also prevent other programs from opening, including any security software already installed on the PC, making web users feel they have no choice but to part with their hard-earned cash.

But once worried users pay up, not only have cybercriminals managed to extract money, they also have users' credit card details.

Google said the number of domains hosting fake antivirus software surged in 2009. At the end of 2009 there 587 domains hosting fake antivirus software compared to just 93 at the start of last year.

"As users are becoming increasingly aware of the need to secure their computers, attackers have been leveraging this awareness by employing social engineering techniques to distribute Fake AV software," Google said.

"Fake AV domains often target high-profile sites. For example, Facebook, the New York Times and Twitter have all been used to distribute Fake Antivirus (often through malicious advertisement or user posts."

Google said it had developed software that will speed up detection of fake antivirus software in a bid to help genuine security software detect fake antivirus code.

See also: Scammers pounce on McAfee fiasco with fake AV