I had installed .NET Framework 4 in my system. It says that .NET Framework 4 is backwards compatible with all previous versions. But an application installer still asks me to install .NET Framework 3.5 SP1.

What should I do to make the installer aware of .NET 4's presence?

Is it necessary to install .NET. 3.5 SP1 too? (It is a big setup, ~250 MB.)