Microsoft does not offer a standalone "Windows Installer 5.0.msi" download for Windows 10 for one simple reason:

: Enhanced options for customizing how services are configured during the installation process. 💡 How to Check Your Version

To unregister the service, type the following command and press : msiexec /unregister

If the above fails, download the from Microsoft, run it, and choose "Upgrade this PC now." This reinstalls the entire operating system while preserving your apps and files. It will completely restore Windows Installer 5.0 to its latest build.