InfoWorld takes over Microsoft Security Essentials’s update mechanism, “If you’ve switched Microsoft Update or Windows Update to “Notify but don’t download” or “Download but don’t install,” running MSE installer zaps out your setting & switches Windows over to Automatic Updates. More to the point, MSE installer does its little bit of mischief without warning or notification or permission. As a result, many people who install MSE are floored to discover — usually after a batch of security patches gets automatically applied — that their carefully considered Automatic Update setting has been trampled.
The resolution is to simply go back into Windows and change it. MSE itself doesn’t flip it back. MSE uses Microsoft Update/Windows Update mechanism to keep itself updated with latest signature files, but it bypasses Automatic Update setting in Windows. If you switch to “Notify but don’t download,” for e.g., MSE will still get updated once a day the way it should.”