Summary: The Rhino installer reports that the .NET 4.0 framework is required.
Related installation help: Troubleshooting Rhino 5 Installation

Problem

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.

Cause

.NET Framework 4 is somehow broken on the system.

Solution

1. Download .NET Framework Cleanup tool.
2. Run .NET Framework Cleanup tool.
3. Select .NET Framework - All Versions for the product to cleanup.
   
4. Click the Cleanup Now button.
   This takes some minutes to complete.
5. If your computer does not reboot automatically, reboot.
6. Install Rhino 5 again.

Note: If this cleanup tool doesn't fix the problem, you can also try the .NET Framework Repair tool.