As well as mining bitcoins, PCs enrolled in ZeroAccess were also being used to poison search results - to cause users to unwittingly click on booby-trapped web pages - or fraudulently click on adverts to generate revenue.
The operators of one of the biggest current botnets, known as ZeroAccess, had recently ramped up their efforts to use machines they control to mine bitcoins, he said, adding that millions of infected PCs were unwittingly enrolled in the criminal network.