The software provides basic anti-virus protection to licensed users of the Windows operating system that automatically updates through the Windows update facility.
More than 1.5 million copies were downloaded in the first week, and a year later there are 30 million users worldwide, said Cliff Evans, head of security and privacy at Microsoft UK.
"Since launch, Security Essentials has achieved wide coverage of 259 locations around the world," he said.
The UK has the third highest number of active users after the US and Brazil, followed by Germany, Japan and France.
"Awareness of Security Essentials is high in the UK, and we are talking to numerous banks about encouraging customers to use it, Evans told Computer Weekly.
The software has expanded from an initial 17 countries and eight languages to 74 countries and 25 languages, but is on track to achieve the target of 83 countries and 33 languages in 2010, he said.
Microsoft recently announced that free licences for Security Essentials will be available for small businesses with up to 10 PCs from October.
Microsoft introduced Security Essentials to improve the overall security of the computer community by offering free lifetime security software without the cost and complication of subscription renewals.
"Many computer users do not understand how the free trials work, and this together with cost has been the main reason not to run security software," said Evans.
But Security Essentials enables any home or small business user of Windows to benefit from a layered approach to security achieved when used in combination with the Internet Explorer (IE) browser, he said.
Smartscreen technology introduced in IE 7 has been refined and enhanced in IE 8 and IE 9 to tackle threats such as phishing attacks that may slip through e-mail monitoring systems, said Evans.
IE will warn users about links to potentially harmful malware, but even if such code is executed by mistake, Security Essentials provides another layer of security to block known threats and identify suspicious behaviour in any new threat, he said.