I did some further testing, to no avail.
Some days ago a new set of GPLPV Drivers were released. I tried to install ejbpv_2003x64_1.0.1105.msi, but I still the same error, which looks like this:
This is with a fresh install of WXP x64 SP2. The only other applications that are installed are WinRAR, Google Chrome, and as a Windows component, .NET Framework 3.5 SP1. After failling to install the latest Drivers, today I also tried with two Hotfixes which seemed promising based on what I googled about the Windows Installer error, so I installed the latest Windows Installer 4.5 upgrade, WindowsServer2003-KB942288-v4-x64.exe, and also a Hotfix to upgrade Windows Script Host from 5.6 to 5.7, WindowsServer2003.WindowsXP-KB955360-v4-x64-ENU.exe:
...and still no go.
Is there absolutely no one that tried Windows XP x64 recently with GPLPV Drivers that can confirm if the installer works or not?