Bump on that.
Have been using Windows Essentials for 2 years, XP Pro machine and Win 7 machine.
Run Malwarebytes and Kaspersky tdsskiller once a month to make double sure.
Microsoft Essentials hasn't let anything through yet.
Works slick and doesn't eat up processor time.
Had McAfee before, just became too much bloatware for the older machine, would almost drag it to a stop.
I started Ms Essentials when I got a Win7 laptop and desktop.
Previously on the XP, I had used Norton, Trend Micro, McAfee, AVG, and AVAST!. They all eventually became bloatware, would slow my system, would conflict with other software, etc.
I do occasionally scan with SUPERAnti-Spyware, and clean the registry with CCleaner and Glary Utilities.