Microsoft Security Essentials is well designed and easy to use. Installation is simple and straightforward, although it will verify whether your copy of Windows is legit along the way. Once you install it and open it, you'll be greeted by a thoughtfully designed main screen. This screen has four tabs: Home (which shows status information, scan controls, and an update button if your virus definitions are out of date); Update; History (which logs all of the malware cleaned from your system); and Settings.

Security Essentials has a green/yellow/red color-coded status bar that runs across the top of the window--as is common in antivirus software--and the Home tab gives you more details as to your PC's protection status.

While not terrible, Microsoft Security Essentials lagged behind the top performers in our recent antivirus roundup at detecting malware using traditional scanner-based detection methods (which rely predominantly on malware definition files), detecting 92.7 percent of samples. This was the second-lowest score of the free antivirus products (Comodo's free Internet Security Premium was slightly worse, detecting 92.4 percent of samples), and is well behind the top performers, which detected over 99 percent of malware samples.

Security Essentials logged the lowest score in tests to see how well it could block real, live malware attacks. In these real-world attack tests, it completely blocked 64 percent of attacks, and partially blocked an additional 8 percent of attacks. No free antivirus product was able to fully block all attacks, but Comodo scored a 96 percent full-blocking rate. This is a good test to determine how well security products can block brand-new, still-unknown malware.

On the other hand, once an infection is on your PC, Security Essentials will do a relatively good job at cleaning it up: It detected all infections on our test PC, and removed all active components of an infection 70 percent of the time--about average for the products we reviewed. And it managed to completely clean up 50 percent of infections--tops among the contenders.

Security Essentials' scan speeds are among the slowest among products we looked at, as well. It completed our on-demand scan test--which simulates how long it will take to manually scan 4.5GB of data--in 3 minutes, 24 seconds, the second slowest performer in this test. The top performer, Avira AntiVir Personal, completed the test in 87 seconds. Security Essentials was also on the slow side in on-access scan speed tests, which judge how well a product can scan files as they're opened or saved to disk: It did the deed in 5 minutes, 41 seconds, a full 2 minutes behind the leader.

Despite the slow scan speed, Security Essentials had a fairly low impact on overall PC performance. It added less than 1 second to startup times in our tests, and finished most of our other system speed tests with better-than-average scores.

There's a lot to like about Microsoft Security Essentials, but we were a little disappointed by its malware detection and attack blocking--after all, antivirus software is only as good as its ability to keep your PC protected. Hopefully, Microsoft will bolster Security Essentials' threat-blocking capabilities in a future update, but until then, you'd be better served by going with something having better detection capabilities, such as Avira AntiVir Personal.