What exactly do you want to put in your window: text? images? controls?
If you want to design a dialog with controls, you can use what's generically called a "resource editor" to place controls (edit entry fields, listboxes, buttons, all those other bells & whistles) in a dialog, which is a "modal" window. You can get these tools for free. (They don't really generate code: they generate data that you can use to create the dialog. You still have to write code to initialize the controls and respond to the user's interaction with them.)
We need a little more information here.