PC Magazine shares an recent review of MSE 4.0 … In personal testing on all family PCs, there is more consistency in auto-updating and it continues to be transparent in terms of impacting performance.   No issues so far in use on XP and Windows 7 systems.

Microsoft Security Essentials 4.0

QUOTE: This product is a straight-up antivirus, not a feature-stuffed suite wannabe. Its main window shows current security status, with a button to resolve any problems. Another button launches an on-demand scan. That’s it!

Pros — Does a decent job protecting an already-clean system. Good ratings from independent antivirus test labs. Free!

Cons — Unusually slow scan. Failed to run on one test system. Low detection rate in malware cleanup test. Failed to thoroughly clean up threats it did detect.

Bottom Line — Microsoft Security Essentials 4.0 does a decent job protecting a clean PC, but in testing its cleanup of already-infested systems wasn’t thorough.