Problems with Windows XP menu effects on NVidia h/w

I'm experiencing a weird artefact with the Windows XP menu fade effect and OpenGL.

The problem occurs when displaying a context menu using MFC CMenu::TrackPopupMenu within an OpenGL rendered window. When I select an item in the menu the window is redrawn but the menu item remains (although the rest of the menu is erased).

It appears that something like the following is occuring:

TrackPopupMenu
		WM_PAINT
		Menu displayed
Select an item
		Menu hidden
		WM_PAINT
		Animated fade effect drawn				

The problem occurs whether I'm double buffered or not and whether SwapBuffers is synced to the vertical refresh of the gfx card. This seems to affect all nVidia hardware but not ATI based kit.

Anyone any suggestions?

Thanks
-
Robin Charlton