MSE was launched I believe last July. I have been using it for past 5 months with Vista and XP. It is free anti-virus/maleware and replaces Defender. MSE automatically disables Defender during installation.
In my experence detection is as good as my previous virus programmes, AVG, Avast and others.
MSE does not effect performance and updates daily.
Windows 10 Home vs Windows 10 Pro comparison: Here are the extra features you get in the Pro version