Hackers have increased their attempts to hijack PCs since the start of the year, with up to 75,000 being compromised daily, according to Symantec.
The security firm's biannual Internet Security Threat Report found that the average number of PCs taken over by remote control was up from 2,000 to 30,000 a day.
Once a PC is compromised, data such as financial details can be harvested and the machine can be used to send spam or attack networks as part of an army of 'bots', or remotely controlled computers.
Richard Archdeacon, director of technical services at Symantec, said in a statement: "Bot networks create unique problems for organisations and individual PC users as systems can be automatically upgraded with new exploits very quickly.
"This allows attackers to outpace efforts to patch or download security updates. We saw a steady increase in the number of bots during the reporting period."
The report also found that the average time from a vulnerability being discovered to the release of exploit code has dropped from seven days to 5.8 days.
In addition 95 per cent of the 1,237 new vulnerabilities discovered between January and June were rated 'highly severe'.
The report claimed that 4,496 new Windows viruses and worms were detected during the same period, four and a half times more than in 2003.
Most of the attacks originated from the US, reflecting the country's level of internet infrastructure.
But as a percentage of population Latvia, Israel and Macau produced more attacks than any other.
Attacks from Japan and South Korea have dropped since last year, indicating that public awareness campaigns held in those countries have had some success.
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