A Bamital botnet raked in an estimated million dollars annually by routing internet users to websites that generated revenue with bogus online ad clicks.
"The Bamital botnet defrauded the entire online advertising platform, which is what allows the internet and many online services to be free," Microsoft said in a blog post.
"What's most concerning is that these cybercriminals made people go to sites that they never intended to go, and took control of the computer away from its owner."
Along with generating fraudulent clicks for which advertisers paid, the hackers sent internet users to websites that could sneak malicious code onto machines or steal personal information, according to Microsoft.
Microsoft and Symantec research found that during the past two years, more than eight million computers were attacked by Bamital, and that the scheme targeted popular search services and browser programs.
Symantec said that it has tracked the botnet since late 2009, and joined forces with Microsoft to shut down the operation.
"Bamital is just one of many botnets that utilise click fraud for monetary gain and to foster other cybercrime activities," Symantec said in a blog post.
"Many of the attackers behind these schemes feel they are low risk, as many users are unaware that their computers are being used for these activities."
Bamital is part of a family of malicious software designed to highjack search engine results and route internet users to hacker-controlled servers, which then redirect traffic to other websites, according to Symantec.