The BBC points us to a rather lengthy 240 page report, published by Microsoft, called the Microsoft Security Intelligence Report. Microsoft collected data throughout the first half of this year and found that the United States has the highest number of computers infected with a virus that places them into a botnet.
The study found that over two million computers in the U.S. were reported as having a botnet virus, while the country ranked second place, Brazil, had around 550,000 computers infected. Microsoft used the reported results from their Malicious Software Removal Tool which is updated and delivered every month via Windows Updates.
Though the number infected was highest in the United States, South Korea had the highest percentage of computers infected. 14.6 out of every 1000 South Korean computers are part of a botnet. The US, comparatively, reported in at 5.2 infected computers for every 1000.
Cliff Evans, head of security and identity at Microsoft UK, said,
The research was undertaken 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. Few people know about botnets. Once they have control of the machine they have the potential to put any kind of malicious code on there. It becomes a distributed computing resource they then sell on to others.
To be sure you aren't infected, you should always keep your computer and your anti-virus up to date. You can also run the Malicious Software Removal Tool from Microsoft. In Windows XP, go to Start -> Run and type "MRT" or in Windows Vista/7 you can type "MRT" in your Start menu search box.