At least one anti-malware vendor is using a form of malware – in this case linked to social engineering – to market its product. 

According to Bhaskar Krishna, writing on the McAfee Avert Labs blog, a potenitally-unwanted program (pup) named Malwarewipe is being marketed by a trojan called Puper.
The strategy begins with Puper dropping its supporting files on a user’s system for further action and then displaying hoax balloon messages informing the user that it has detected malware on the computer and inviting him to download security software to fix the problem.
If these early warnings are ignored, more serious-looing warnings will be displayed, warning the user about system vulnerabilities.
Other messages, purporting to come from Windows, contain technical details about the supposed infections and urge users to click on a button to download a fix.
Once a user gives in an clicks the button, the trojan redirects the browser to a MalwareWipe page where users have the opportunity to purchase the product.