using a wpf window in a class library project

Is it possible to use window (namespace System.Windows) in a library project?

I've tried to use some wpf controls in a library project, but the only control I can add is the UserControl.xaml (and I need to show some windows programmatically).

I've also tried to copy-paste a window into the dll project but I received the error:

Library project file cannot specify ApplicationDefinition element.

thanks

GHillo