Thread: Getting the position of a child window

Hi,
I'm trying to process the WM_SIZE message in my main window's WindowProc so that a child window (used for displaying images) has a scrollbar window underneath it (not actually "in" the child window). I've tried using GetWindowRect and setting the scrollbars window's Top value to the resulting RECT's top, but as the coordinates are relative to the screen this doesn't work properly. I was thinking of perhaps using MapWindowPoints in some way. Any ideas?