With Windows Installer 4.5, we had an MSI install which wouldn’t complete the user section of the install.

Symantec_fail And it turned out to be a fault with Windows Installer 4.5.

The installation process of a MSI package that contains multiple packages stops responding (hangs) in Windows XP, Windows Vista, or Windows Server 2008

We need to use Windows Installer 4.5, as we’re seeing some software installs which require it as a prerequisite.  So we need the hotfix.

But, this from the company “where good products go to die”:

Some Microsoft Windows Installer hotfixes may have error handlers that do not ignore any errors and fail the installation process if any errors are encountered.

One such hotfix is Microsoft Windows Installer hotfix KB981669. If Microsoft hotfix KB981669 is installed, the hotfix will not ignore any errors and will cause the PGP Desktop installation to fail.
(my emphasis)

Alternately, they could have fixed their PGP Desktop installation, so it doesn’t produce errors.