With .NET 4.0, installing a new version of .NET 4.0 has no effect on existing applications built in earlier versions of .NET
Components that were built with the .NET 1.1 can run side by side with version 2.0, 3.0, or 3.5 components
When native application that uses COM components built with earlier versions of the .NET Framework, the COM components will run with the version of the .NET Framework they were registered with
Managed application built with the .NET Framework 3.0 SP1 will run with the .NET Framework version 3.0, but can be configured to run on the .NET Framework 4 if version 3.0 is not present