Solved. Thank you for your help.
It turns out the SourceForge and OSDN downloads were identical. OSDN showed the timestamp as 2018. I thought it was a new build of the old version. :-)
For anyone searching this in the future, the exact error message was "This installation package cannot be installed by the Windows Installer Service. You must install a Windows Service Pack that contains a new version of the Windows Installer service."
Installing Windows Installer 4.5 Redistributable fixed it. The system is XP SP3. I only keep this XP machine for some old irreplaceable (for now) programs that don't play nice with newer Windows. It never connects to a network!!