I wrote a tutorial years ago about creating a Windows screen saver using the C# language. I usually let my GUI students do it as an extra credit assignment.
The tutorial was really showing signs of age, and last week I was finally able to update it. You can find it here. If you'd prefer to just jump straight to the finished product, you can download the project here. I used Visual Studio 2010 to write the demo code.
One more thing to cross off my summer to-do list.