The number of computers hijacked by malicious hackers to send out spam and viruses has grown almost 30% in the last year, according to a survey.
More than six million computers world wide are now part of a "bot network", reported security firm Symantec.
Computer users typically do not know that their PC has been hijacked.
More than a third of all computer attacks in the second half of 2006 originated from PCs in the United States, the threat report said.
While the total number of bot-net PCs rose, the number of servers controlling them dropped by about 25% to 4,700, the twice-yearly report said.
Symantec researchers said the decrease showed that bot network owners were consolidating to expand their networks, creating a more centralised structure for launching attacks.
Ollie Whitehouse, senior consulting services director at Symantec, said: "This rise in the number of infected computers can certainly be attributed to the rise in the online population of countries like China and Spain, in Europe.
"There is almost an educational curve that the users and service providers have to go through. Unfortunately when certain countries go through rapid increases in connectivity and availability of technology that curve is not always kept up.
Alfred Huger, vice president of Symantec Security Response, said online criminals appeared to be adopting more sophisticated means of "self-policing".
He added: "They're launching denial-of-service attacks on rivals' servers and posting pictures online of competitors' faces.
"It's ruthless, highly organised and highly evolved."
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