Visual Studio 2008 is the first version of the IDE that allows you to target the version of the .NET Framework that you want to work with. When you open the New Project dialog and get ready to create
a new project, you will notice that there is a drop-down list in the upper right-hand corner of the dialog that allows you to pick the version of the framework that you are interested in using. This dialog is presented.
From this figure, you can see that the drop-down list provides you the ability to target the .NET Framework 2.0, 3.0, or 3.5.This is possible only because the 3.0 and 3.5 versions of the framework are extensions of the .NET Framework 2.0. When you use the upgrade dialog to upgrade a Visual Studio 2005 solution to Visual Studio 2008,it is important that you are only upgrading the solution to use Visual Studio 2008 and that you are not upgrading your project to the .NET Framework 3.5.Your project will stay on the framework version you were using, but now, you will be able use the new Visual Studio 2008 to work on your project.
If you want to change the version of the frameWork the solution is using, right-click the solution and select the properties of the solution, If you are working with an ASP,NET project, you will get a dialog as shown.
From this dialog, the Build tab will provide you the ability to change the version of the framework that the application is using, If you are working with a Windows Forms application’s property pages, you will find the ability to target another version of the framework on the Application tab (the first tab). This is presented.