Vint Cerf says 1 in 4 computers are secretly controlled by hackers:

Of the 600 million computers currently on the internet, between 100 and 150 million were already part of these botnets, Mr Cerf said.

Botnets are made up of large numbers of computers that malicious hackers have brought under their control after infecting them with so-called Trojan virus programs.

While most owners are oblivious to the infection, the networks of tens of thousands of computers are used to launch spam e-mail campaigns, denial-of-service attacks or online fraud schemes.