Default .net framework reference path ?

Hi,

I create VS extensibility and I need to add a .net framework libraries to a project. I check in VS behaviour that project target .NET Framework 2.0 when add reference will show .net framework 2.0 libraries from C:\Windows\Microsoft.NET\Framework\v2.0.50727 and when I change to 4.0 it retrieve from C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.0.

I want to know where to get these information ? I check in registery HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework but can not find it.

It's hard to be advanced programmer