More than 1.5 million Windows users downloaded Microsoft's free anti-virus and anti-malware tool, Security Essentials in the week after it was released, the software firm has claimed.
Almost four million pieces of malware were detected on 535,752 machines in the same period, according to a Microsoft blog post.
The company's Security Essentials software provides basic anti-virus protection to licensed users of the Windows operating system not covered by other security software.
"In one week, Microsoft Security Essentials is making a big difference to those people using it on their computers," the blog post said.
The free software has proved most popular with business users of the Windows 7 operating system, which is due for consumer release later this week.
According to Microsoft, 44% of Security Essentials users are running Windows 7, followed by XP (33 %) and Vista (23%).
Fifty two per cent of malware detected in the first week was found on machines running XP, followed by Vista (32%) and Windows 7 (16%).
"This follows our usual observed trend of seeing less malware on newer OS and service packs," the blog post said.