Did you download the installer directly from Microsoft's website? I think this problem relates to anti-virus software, if you have it disable it and then try to run the installer. Also if you didn't try it already, maybe a reboot would help.
No, it has to do with the installer using a old or expired Certificate. The new Windows 8 probably uses a different chain or root certificates. You can try downloading and installing the root certificate for that particular Installer.
Majority of people get their certficates from Symantec (Formerly Verisign).