Yeah, I had the same SmartScreen warning message as well (on Win8.1). This warning has not come up with other unity made games, so there does seem to be something different about your build. If I remember correctly, I had the same thing happen with Lieve Oma.
In the warning dialogue box, I was able to chose the "Run Anyway" option, which I thiiiink showed up after choosing "More info."
edit: spelling n stuff