Microsoft Security Essentials Beta

The app's ability to detect and block malware was neither especially good nor particularly bad. Its 97.8 percent overall detection rate put it in fourth; but it did well in proactive tests, which use two- and four-week-old signature databases to simulate how well a program detects new, unknown malware. Its results of 52 percent and 43.8 percent, respectively, were second only to those of the top-ranked Avira AntiVir Personal, our overall winner.

Microsoft's program put up no false positives (flagging of benign software), and it got a near-perfect score overall in detecting and cleaning rootkits and malware infections. It detected and disabled every infection, and although it left behind several changes to the Registry and other areas (as every free app did), they couldn't cause further harm.

Security Essentials has a pleasing interface, is simple to use, and has appropriate defaults. Its warning pop-ups let you deal with an issue quickly or dig in for details. If Microsoft can improve the detection rate a bit--and rev up the scan speed more than a bit--before the program's final release, Security Essentials could turn out to be a real contender in the free antivirus arena.