"!WinForms" is the common nickname for the Microsoft Windows Forms Library, which is also available in NovellMono. The common namespace for these classes is System.Windows.Forms, although there are related namespaces such as System.Drawing. Also, this library calls a "window" a "form" (even though form fields are not required to be present).

For more information, see the Cobra How To: http://cobra-language.com/how-to/WinForms