How Do Rogue Antiviruses Spread

Rogue security software mainly relies on social engineering (fraud) in order to defeat the security built into modern operating system and browser software and install itself onto victims' computers. A website may for example display a fictitious warning dialog stating that someone's machine is infected, and encouraging them to install or purchase scareware.
Most have a Trojan horse component, which users are misled into installing. The Trojan may be disguised as:

• A browser plug-in or extension (typically toolbar)
• An image, screensaver or archive file attached to an e-mail message
• Multimedia codec required to play a certain video clip
• Software shared on peer-to-peer networks
• A free online malware scanning service

Some rogue security software, however, propagate onto users computers as drive-by downloads which exploit security vulnerabilities in web browsers, pdf viewers, or e-mail clients to install themselves without any manual interaction.
More recently, malware distributors have been utilizing SEO poisoning techniques by pushing infected URLs to the top of search engine results about recent news events. People looking for articles on such events on a search engine may encounter results that, upon being clicked, are instead redirected through a series of sites before arriving at a landing page that says that their machine is infected and pushes a download to a "trial" of the rogue program.