The origin is the top left corner of the primary monitor. If you have placed a monitor to the left of the primary monitor, the x-coordinates on that monitor will be negative. If a monitor is above or partially above the primary monitor, they y-coordinates will be negative in that area.
GetSystemMetrics also has constants that refer only to a Multiple Monitor system. SM_XVIRTUALSCREEN and SM_YVIRTUALSCREEN identify the upper-left corner of the virtual screen, SM_CXVIRTUALSCREEN and SM_CYVIRTUALSCREEN are the vertical and horizontal measurements of the virtual screen
For more see here:
For your task I think you probably only need to think about the virtual screen coordinates.