C / C++ / MFC

#include <multimon.h>

        if ( WinPlatform() & ( WIN_98 | WIN_NT5 )
          && GetSystemMetrics( SM_CMONITORS ) > 1    // if > one monitor.
           )
        {
            // SM_CXVIRTUALSCREEN, SM_CYVIRTUALSCREEN  are the width and height,
            // in pixels, of the virtual screen.
            // SM_XVIRTUALSCREEN, SM_YVIRTUALSCREEN metrics are the coordinates
            // of the top left corner of the virtual screen.
            // The virtual screen is the bounding rectangle of all display monitors.
            // See also: SM_CMONITORS, SM_SAMEDISPLAYFORMAT.
            pRect->top    = GetSystemMetrics( SM_YVIRTUALSCREEN );
            pRect->left   = GetSystemMetrics( SM_XVIRTUALSCREEN );
            pRect->right  = GetSystemMetrics( SM_CXVIRTUALSCREEN ) + pRect->left;
            pRect->bottom = GetSystemMetrics( SM_CYVIRTUALSCREEN ) + pRect->top;
        }