Summary: The Rhino installer reports that the .NET 4.0 framework is required.
Related installation help: Troubleshooting Rhino 5 Installation
The Rhino 5 installer keeps asking for you to install .NET Framework 4, but .NET Framework 4 already exists on the system. Rhino 5 is not allowed to install.
.NET Framework 4 is somehow broken on the system.
Note: If this cleanup tool doesn't fix the problem, you can also try the .NET Framework Repair tool.