That's Microsoft's "SmartScreen". It's been in Internet Explorer since version 8. It's just now more prominent in Windows 8. All it is saying is that the program you've downloaded is not signed with a Microsoft certificate (common among small developers like myself), and Microsoft has not specifically reviewed my website and/or application and marked it as "trusted" (also common among small developers like myself). It doesn't mean it has detected anything dangerous. Just press "Run anyway".