AVG, McAfee, Sophos, and Trend Micro claimed that the integration of the product into Microsoft Windows would be a violation of competition law.

The product received generally positive reviews, praising its user interface, low resource usage, and freeware license.

[14] MSE provided real-time protection, constantly monitoring activities on the computer, scanning new files as they are created or downloaded from the Internet.

If no response was received within ten minutes, suspected threats are handled according to the default actions defined in the application's settings.

[17] Hardware requirements for the product depended on the operating system; on a computer running Windows Vista or Windows 7, it requires a 1 GHz processor, 1 GB of RAM, a computer monitor with a display resolution of at least 800 × 600 pixels, 200 MB of free hard disk space and a stable Internet connection.

[21] On 23 June 2009, Microsoft released a public beta to 75,000 users in the United States, Israel, China and Brazil.

A public beta program started on 18 November 2011, when Microsoft sent out invitations to potential participants without announcing a version number.

[16] Microsoft subsequently initiated a pre-release program that provides volunteers with the latest beta version and accepts feedback.

This update fixes a bug that was introduced earlier in version 4.10.205.0 which removed the "Scan with Microsoft Security Essentials" entry from the right-click context menu on files and folders.

Although support for Windows 7[36] ended on 14 January 2020 Microsoft will continue to update virus definitions for existing users until 2023.

On 13 September 2011, at the Build conference in Anaheim, California, Microsoft unveiled the developer preview of Windows 8, which had a security component capable of preventing an infected USB flash memory from compromising the system during the boot process.

In an included video, Jason Garms of Microsoft showed how Windows Defender was registered with Action Center as an AV and spyware protection tool, and how it blocks drive-by malware.

[44] Jens Meggers, Symantec's vice president of engineering for Norton products, dismissed it as "very average – nothing outstanding".

[46] Joris Evers, director of worldwide public relations for McAfee stated "with OneCare's market share of less than 2%, we understand Microsoft's decision to shift attention to their core business.

Vincent Steckler, Avast Software CEO wrote in a blog post "MSE is not the silver bullet but it is also not the bad sequel to One Care [sic] that some claim.

[49] McAfee, Sophos and later Trend Micro affirmed that an antitrust lawsuit would surely have followed if Microsoft had bundled the product with Windows.

Daniel Ives, an analyst with FBR Capital Markets, said that Microsoft Security Essentials would be a "long-term competitive threat", although near-term impact would be negligible.

[52] The public beta version received several reviews, citing its low resource usage, straightforward user interface and price point.

[15] Ars Technica reviewed it positively, citing its organized interface, low resource usage, and its status as freeware.

[63] In October 2012, the product lost its AV-TEST certification when Microsoft Security Essentials 4.1 achieved scores of 1.5, 3.5 and 5.5 for its protection, repair and usability.

He named Microsoft Security Essentials as an influence on PC users to adopt free AV software.

[73] Seth Rosenblatt of CNET News commented on how the product's share rose from 7.27 in 2010 to 10.08 in 2012, stating that "use of the lightweight security suite exploded last year".

Designated Rogue:Win32/FakePAV or Unknown Win32/Trojan, it closely resembles Microsoft Security Essentials and uses sophisticated social engineering to deceive users and infect their systems, under the guise of five different fictional anti-malware products.