Visual Studio 2008 allows you to switch which version of the .net framework your project targets.  The available options are 2.0, 3.0, and 3.5. 

To change the framework version open up your project properties compile tab

You will find the framework option combobox on the form that opens when you press the Advanced Compile Options button

Visual studio will save the project, close, and finally restart with project opened up when you change the framwork version for your project