It's a multiple documents interface program (where one main window contains subwindows), a UI style out of fashion since the early days of Windows 95 (IIRC it was Office 97 that switched Office apps from MDI to multiple main windows for example). For some reason Microsoft hasn't updated the theming for them since Windows Vista, probably because it's such a niche thing. Even on Vista / 7, they're stuck as Aero Basic windows, rather than the actually glass ones. The Windows Forms editor in Visual Studio also using those window borders, which shows how much MS cares about that compared to XAML dialect of the week.
The Aero Basic theme is particularly horrible though, I do wonder if that particularly unpleasant shade of cyan for the borders was picked to punish those who didn't activate Windows, or were to poor to afford Home Premium.
In the end it's one of those weird forgotten things, like the Windows 3.x colour picker dialogue that still lurks in a few places like Wordpad (although MS are "solving" that).