If none of the above works, ask the vendor which exact version of the InstallScript engine was used to build their setup. They may provide the correct engine file directly.
Sometimes the Windows Installer itself is unregistered. You can reset it using the Command Prompt: isscriptmsi new download
Unlike many common Windows components, ISScript.msi is hosted on official Microsoft servers. Instead, the most reliable, up‑to‑date source is the InstallEngine website , which has been maintained by the InstallSite community for years. If none of the above works, ask the