The computer software maker said Wednesday the United States leads the world in the number of Windows PCs that have been drawn into botnets, networks of computers controlled remotely generally for a malicious purpose.
High-tech criminals use botnets to send out spam, phishing emails and launch attacks on Websites.
They are also used to gain information like passwords for online bank accounts that can later be resold.
In a 240-page report Microsoft said its research was undertaken to alert people to the growing danger from malicious networks.
The company found that Brazil has 550,000 infected computers while 14.6 out of every 1,000 computers in South Korea are part of botnets.
"Few people know about botnets," said Cliff Evans, head of security and identify at Microsoft UK.