Windows 8.1 won't install MM

[Liana5]

I am trying to download and install (Lifetime License) on a new computer with Windows 8.1. At first, it doesn't want to download because it's an unknown app, etc. But, got it to download anyway. Twice.

The first time I tried to install it said a file was corrupt.
The second time it said there was a side-by-side error.

What shall I try next?

[Peke]

MMW is not unknown app it have valid digital sugnature.