Two million US PCs recruited to botnets

More than 2.2 million US PCs were found to be part of botnets, networks of hijacked home computers, in the first six months of 2010, it said.

Compiled by Microsoft, the research revealed that Brazil had the second highest level of infections at 550,000.

Infections were highest in South Korea where 14.6 out of every 1000 machines were found to be enrolled in botnets.

The 240-page Microsoft report took an in-depth look at botnets which, said Cliff Evans, head of security and identity at Microsoft UK, now sat at the centre of many cybercrime operations.

The research was undertaken, he said, to alert people to the growing danger from the malicious networks.

"Most people have this idea of a virus and how it used to announce itself," BBC News quoted him as saying. "Few people know about botnets."

Hi-tech criminals use botnets to send out spam, phishing e-mails and launch attacks on websites. Owners of botnets also scour infected machines for information that can be sold on the underground auction sites and markets found online.

Botnets start when a virus infects a computer, either through spam or an infected web page. The virus puts the Windows machine under the control of a botnet herder.

"Once they have control of the machine they have the potential to put any kind of malicious code on there," said Mr Evans. "It becomes a distributed computing resource they then sell on to others."