Windows 8 Certificate Warning: We can't verify who created this file
Perhaps this is a bit late, but I figured I'd post this in case others arrive at this article with the same problem. It seems that Microsoft, in 2013, made a change to Windows such that executables opened from a network drive will ALWAYS give this warning, even if they are signed. Source (it is mentioned in the comments section) : http://blogs.msdn.com/b/ieinternals/archive/2011/03/22/authenticode-code-signing-for-developers-for-file-downloads-building-smartscreen-application-reputation.aspx?PageIndex=2)
It seems the only way to make this go away is to do what user4437298 suggested, which is to add the network drive as a trusted zone.
Add your z:\ drive to trusted sites in internet explorer local intranet advanced options