Microsoft announced Thursday that its free antivirus software has been downloaded 1.5 million in the week after it shipped.

The company's Security Essentials software is a basic antivirus program designed for Windows users who don't want to pay $40 to $50 per year for an Anti-Virus software.

Microsoft's free AV was launched on Sept. 29, and by Oct. 6, the company said it had been downloaded more than 1.5 million times, according to a Microsoft blog posting

Microsoft recorded 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.

Security Essentials is currently available in 19 countries.

Click here to download Microsoft Security Essentials.