How can I create an animated splash screen like the one in Office 2010 using C#?
Is this question about winforms or wpf?
If it's about wpf:
An animated splash screen is not more than a wpf window showing while your "Main Window" is loading. You can design this splash window with Expression Blend as explained by wischi.
You can also have a look at this code project.
For creating some kind of a loading animation: A Simple WPF Loading Animation
Just create a window with an animation defined in xaml and show it while your application is loading -> animated splash screen.
You may have to override the paint method of a form to create an animation. But it's still showing another window which contains an animation while another window is loading.