Windows 8 and running the pre-race update...

oh crap :-/

something must be special about your internet connection then :-\

Maybe you use a proxy server or so?

 

alright, lets debug that manually, navigate to this folder on your computer (after the setup.exe broke):

C:\Users\ < YOUR USERNAME HERE> \AppData\Local\BeamNG\BeamNG.drive\alpha_prerace

and download this zip file: http://www.beamng.com/attachment.php?attachmentid=32620&d=1403575615

and unpack its content to that folder.

Then run launcher.exe

This should leave you with a running game. Sorry that it didn't work out on the first try, but its really hard to test with all scenarios.