As a last resort, the installer can be executed even though it's corrupted. To bypass the corruption test, pass /NCRC on the command line. To do so, go to Start->Run, type cmd and hit Enter. A black console window should open. Drag the installer into the window, hit Space and type /NCRC. Hit Enter again and the installer should start. This is not recommended!
When all else fails, you will need to contact either the developers of the software you're attempting to download or your personal computer support channels.
To anyone interested,this trick solved my problem. The makers of the game were not interested, once they have sold you the game, job done. This took research of perhaps twenty minutes, why couldn't DCS at least try to solve the problem. Pissed off .