Microsoft's latest Security Intelligence Report indicates that the US is the biggest home for botnets in the world.
More than 2.2 million American PCs are believed to have been infected during the first half of this year. Brazil had the second-highest number of infections, at 550,000.
But South Korea had the highest proportion of infected computers, at 1.46 percent.
The figures are based on the 6.5 million clean-ups that Microsoft performed during the half. During the period, Win32/Rimecud was by far the most commonly-detected bot, with Microsoft clearing up more than three and a half million machines.
There's good news for Windows 7 users, who have lower infection rates than those using earlier versions of Windows.
"The botnet infection rate for Windows 7 and Windows Vista is significantly lower than that of their desktop predecessor Windows XP with any service pack installed, which reflects the security improvements that have been made to the more recent versions of Windows," says the report.
It says that, of those machines that have had the most recent service pack for their operating systems installed, the infection rate for Windows XP SP3 is twice as high as that of Windows Vista SP2 and more than four times as high as that of the release-to manufacturing (RTM) version of Windows 7.
"The good news is there are clear indications that aggressive, creative disruption efforts by the software industry, law enforcement agencies, government entities, and academics are having an impact on botnets," says Adrienne Hall, general manager of Microsoft Trustworthy Computing.
"We’ve seen successful botnet takedowns against Waledac, led by Microsoft and the Mariposa bot arrests, led by Spanish authorities; between April and June 2010, Microsoft cleaned botnet infections from more than 6.5 million computers worldwide; the number of industry disclosed vulnerabilities continues to decline – by eight percent in the second quarter of 2010 compared the previous three months; and since 2006, we’ve seen a 75 percent increase in people using Microsoft’s automatic update service."
The report comes as the company releases its largest-ever security update, plugging 49 vulnerabilities.
The Security Intelligence Report is here.