These are usually anti-spyware or security software applications that use various forms of deception and/or unethical means or show a history of negligent false positives to goad the end user to make a purchase.
In some cases these applications maybe downloaded with some form of unwanted software at which point the rogue application is offered to the customer as a way to remove the unwanted software.
Virusblast is an application that may be installed by a trojan downloader without the concern of the user. This application is promoted on websites like http://protectionbar.com/remove_spyware/ that recommend the use of other rogue security programs. It hooks Internet Explorer and causes Internet Explorer to delay opening for several minutes.