When executing the setup.exe, go to %TEMP% folder and there will be a XMLpad temp installation folder. In that folder setup will extract WMhelp XMLPad.msi, this you can run by: msiexec /i WMhelp XMLpad.msi /qb! And presto!
Do remember that this program require admin rights to run!
Setup Type: Windows Installer (Delivered as an EXE)
Deployment Method Used: Windows Installer Command Line (No MST)