The Xamarin universal installer (from https://xamarin.com/download or the big blue button on https://store.xamarin.com/account/my/subscription/downloads) has just been re-signed with a new certificate because the old code signing certificate had expired. As a result, Windows will now display the following SmartScreen warning screens until a certain number users have downloaded the new installer via Internet Explorer.
Windows protected your PC
Windows SmartScreen prevented an unrecognized app from starting. Running this app might put your PC at risk.
Publisher: Xamarin Inc.