Early reports suggest that the security software - which is Microsoft's replacement for Windows Live OneCare, a pay-for security suite that was retired at the end of June - is being downloaded by a large number of Windows users.
Microsoft offered a beta version of Security Essentials - earlier code-named Morro - to about 75 000 users in the USA in June, but rapidly stopped the distribution after large numbers of people expressed interest.
Earlier this month, Microsoft emailed its beta testers to thank them for their assistance and said that the finished version of the software would be released "in the coming weeks".
Early reviews on the web suggest that the software consumes less memory and disk space than some commercial security suites - and has been commended by AV-Test GmbH, the independent test labs.
Reports are also generally positive from the IT security industry.
The Techradar newswire quoted David Emm, the veteran IT security consultant - now with Kaspersky Lab UK and previously with Dr Solomon's and McAfee - as saying that free is an impressive price.
Emm told the newswire that the software will have a big impact on the IT security software marketplace, but that there is "still room in the industry for quality security software".
"It would be naive to say it doesn't have any implications", he said, adding that "it will depend, I think, on how significant the impact will be".
"We're not immediately concerned; if you look at the developments in the last 10 years or more in anti-virus software it has gone through several iterations."
Over at Microsoft, meanwhile, Amy Barzdukas, the company's general manager for consumer security, said that users have told the software giant that they want the protection of real-time security software, but "we know that too many are either unwilling or unable to pay for it, and so end up unprotected".
"With Microsoft Security Essentials, consumers can get high-quality protection that is easy to get and easy to use - and it won't get in their way", she said.
Microsoft Security Essentials - which is available for Windows XP SP2 or SP3, Windows Vista and Windows 7 including XP mode on both x32 and x64 PCs - is supported in eight languages and 19 countries.