Perhaps you still don't know how to change the 'Target Framework' for your project in Visual Studio. I am using Visual Studio 2008 currently, so the .NET framework should be 3.5 but I already upgraded it to 3.5SP1 ^^
So when I add a new project, I can choose 3 types of .NET framework including 2.0, 3.0 and 3.5. See below screen shot.


Well, I deliberately choose 2.0 first so later I will change it to 3.5 during development time, urm.. maybe I want use some features in 3.5 that can't be found in 2.0 eg. LINQ.

How to change it?
1. Go to Project properties.
2. If you are using C# project, you can directly see this text 'Target Framework:' in application section. Otherwise if you are using VB project, you have to move to Compile section and find a button called 'Advanced Compiled Options...' and click it, you'll get a windows form and you'll also see this text at below 'Target framework (all configurations):', change it to 3.5.
3. You will be prompted a message to ensure changing the target framework.
4. If you click yes, the target framework has been successfully changed.
See below screen shots.