Clickbot.A

Clickbot.A is a botnet that is used for click fraud.
The bot was first discovered by Swa Frantzen at SANS' Internet [Storm Center] in May 2006. At that time, the botnet had infected about 100 machines. The infected population grew to over 100,000 machines within one month.
The bot was written as a plugin to Internet Explorer that was downloaded by IE users. It operated by using victims' computers to automatically click on Pay [per click|pay-per-click] Internet advertisements.
It is also used to steal passwords from unsuspecting users.