Visual studio 2010 : Framework Visibility
Visual Studio 2010 includes only the .NET Framework 4. To target earlier versions of the .NET Framework, you must have the .NET Framework 3.5 Service Pack 1 (SP1) installed. The .NET Framework 3.5 SP1 includes the .NET Framework 2.0, the .NET Framework 3.0, and the .NET Framework 3.5 SP1. To download and install the .NET Framework 3.5 SP1, see Microsoft .NET Framework 3.5 Service Pack 1 on the Microsoft Download Center Web site.
A .NET Framework profile is a subset of the .NET Framework that provides a limited set of libraries and features. An example of a Framework profile is a client profile that can be used to create client applications. For more information about the client profile, see .NET Framework Client Profile.
You can specify the target .NET Framework version or profile by selecting it in the New Project dialog box when you create a project. The list of available project templates is filtered based on the selection.