You have also to take in account the difference between GetDC() and GetWindowDC(). GetDC() retrieves a device context for the client area only. GetWindowDC() retrieves a device context for the whole window including title bar, menus and scroll bars.
If you want the window without the titlebar you have to take for example y-coordinate 0 if you got the device context via GetDC(), if you got it via GetWindowDC() you will have to take 0 + the height of the titlebar and menus.