The company's Security Essentials software is a basic antivirus program designed to appeal to Windows users who don't want to shell out the US$40 to $50 per year that most AV vendors charge. It was launched on Sept. 29, and by Oct. 6, the software had been downloaded more than 1.5 million times, according to a by Microsoft.

The free AV software has proved popular with Microsoft's Windows 7 operating system, which is available to business users now, but set to be made widely available next Thursday. According to Microsoft, 44 percent of users are running Windows 7, followed by XP (33 percent) and then Vista (23 percent).

Though XP is not the most popular platform for Security Essentials, it's where the software is doing the most work. Microsoft counted 4 million total malware detections on more than 500,000 machines during the one-week period; 52 percent of them were on XP machines. Vista was next, with 32 percent of detections, followed by Windows 7, with 16 percent. "This follows our usual observed trend of seeing less malware on newer OSes and service packs," Microsoft said.

The U.S., China and Brazil were the top three countries reporting malware detections, with more than a quarter of all detections occurring in the U.S.