Solving error “Microsoft SQL Server 2008 Setup requires Microsoft .NET Framework 3.5 SP1 to be installed” when you have dotnet .NET 4.0

Don't follow the advised on the TechNet, MS Connect and MSDN forums, they are time-consuming and don't work if you have a newer version of .NET installed like .NET 4.0. Instead, read this and easily fix this hard-to-resolve error by simply changing one registry value — read article