Reviews of the public beta were mostly positive, citing its low resource usage, straightforward user interface, and price point. Brian Krebs of The Washington Post found MSE used 4 megabytes of RAM during testing, even during scans. A "quick scan" took about 10 minutes, and a "full scan" about 45 minutes on an installation of Windows 7.

PCWorld noted its "clear-cut" and "cleanly designed" tabbed user interface. At the top of the main tab, the security status is clearly shown. The other three tabs allow users to manually update MSE, review its history, and change program settings. However, PCWorld found some of the settings to be cryptic and confusing. Settings, such as what to do when malware is found, default to "Microsoft Security Essentials' recommended action". There is no explanation of the recommended action except in the help file. The editor was also confused because MSE does not mention it automatically updates itself within the interface; some may believe they must manually update MSE through the "Update" tab. However, this was included in the final release.

PC Magazine cited MSE's small installation package (about 7 MB, depending on the operating system) and its speedy installation. On the downside, the full installation occupied about 110 MB of disk space, and the initial update took 5 to 15 minutes. The editor also noted the fact MSE sets Windows Update into its fully automatic mode, which automatically downloads and installs updates although it can then be turned off again through the control panel. Installation succeeded on 12 malware-infected systems. Some full scans took over an hour on infected systems; however, a scan on a clean system took 35 minutes.

During an on-demand scan, MSE beta found 89 percent of malware samples; but only 30 percent of commercial keyloggers. Those results were average, according to the editor. MSE found 67 percent of rootkits. The suite detected half of the editor's scareware samples. The suite's real-time protection found 83 percent of malware and blocked the majority of them. In this test, 40 percent of the commercial keyloggers were found. MSE found 78 percent of the rootkits. The editor expressed optimism MSE would improve during its beta testing period.