Security Essentials is a basic anti-virus programme designed to appeal to Windows users who don't want to pay for anti-virus tools. It was launched on 29 September, and by 6 October, the Microsoft Security Essentials had been downloaded more than 1.5 million times, according to a blog post by Microsoft.
That number jumped to 2.6 million in the first two weeks - an "amazing response" to the Security Essentials product, Microsoft said in its blog postings.
Interestingly, the software giant reported that Security Essentials has been popular with early Windows 7 users - business users have had access to Windows 7 for some time, although consumer users will have to wait until later this week for their copies - with 44% of users downloading Security Essentials noted as running the new operating system.
33% of Security Essentials downloads were to Vista-based systems, whilst 23% of downloads were to Vista users.
Microsoft said it counted 4 million total malware detections on more than 500 000 machines during the one week period.
52% of them were on XP machines. Vista was next, with 32% of detections, followed by Windows 7, with just 16%.
"This follows our usual observed trend of seeing less malware on newer OSes and service packs", Microsoft said in a statement.
Despite the success of Security Essentials, many online reviews have been less than charitable, with many comments made about the slowness of the initial malware scan by the application, although Microsoft has said publicly it expects to speed up the scanning process with a new release quite shortly.