Symptoms

Installation in Windows fails immediately upon start.

In installation logs, you find error 1601:

MainEngineThread is returning 1601 MSI

Cause

Error code 1601 indicates an issue with the Windows Installer service:

The Windows Installer service could not be accessed.

You can verify that you are facing this exact issue by trying to install another software using its MSI installer.

Solution

Check the state of the Windows Installer service and start it if it is stopped.

Re-register the Windows Installer service.

See these articles for details:

• Microsoft TechNet. Windows 10 Troubleshooting: The Windows Installer Service could not be accessed 
• Misrosoft Support. "The Windows Installer Service Could Not Be Accessed" error when you try to install a program in Windows 7 or Windows Vista