Since the installer explicitly asks for version 1.0, we will give it version 1.1 (which is nearly identical and satisfies the check).
The engineers at Steinberg in 2009 assumed that Windows would always maintain backward compatibility for .NET 1.0 indefinitely. They wrote a simple if (version < 1.0) then block statement. They never anticipated that Microsoft would stop shipping .NET 1.0 altogether 15 years later. Since the installer explicitly asks for version 1
When the Cubase 5 installer runs, it performs a simple registry check. It looks for a specific key that indicates .NET 1.0 is present. If it doesn't find that exact key, it throws the error. They never anticipated that Microsoft would stop shipping
Surprisingly, enabling .NET 3.5 often tricks the Cubase 5 installer into working because .NET 3.5 includes backward compatibility shims for 1.0 and 1.1. If it doesn't find that exact key, it throws the error
So, when the installer checks your system, it sees:
Install Service Pack 1 (same compatibility mode).
Check the box for .NET Framework 3.5 (includes .NET 2.0 and 3.0) .