Às vezes fica melhor se uma nova janela ou caixa de diálogo é exibida centralizada em relação à janela pai. Esta função faz isso por você. simplesmente chame CenterWindow (hwndChild, hwndParent);.

BOOL CenterWindow(HWND hwndChild, HWND hwndParent)
{
    RECT    rcChild, rcParent;
    int     cxChild, cyChild, cxParent, cyParent;
    int     cxScreen, cyScreen, xNew, yNew;
    HDC     hdc;
 
    GetWindowRect(hwndChild, &rcChild);
    cxChild = rcChild.right - rcChild.left;
    cyChild = rcChild.bottom - rcChild.top;
 
    GetWindowRect(hwndParent, &rcParent);
    cxParent = rcParent.right - rcParent.left;
    cyParent = rcParent.bottom - rcParent.top;
 
    hdc = GetDC(hwndChild);
    cxScreen = GetDeviceCaps(hdc, HORZRES);
    cyScreen = GetDeviceCaps(hdc, VERTRES);
    ReleaseDC(hwndChild, hdc);
 
    xNew = rcParent.left + ((cxParent - cxChild) / 2);
    if (xNew < 0)
    {
        xNew = 0;
    }
    else if ((xNew + cxChild) > cxScreen)
    {
        xNew = cxScreen - cxChild;
    }
 
    yNew = rcParent.top  + ((cyParent - cyChild) / 2);
    if (yNew < 0)
    {
        yNew = 0;
    }
    else if ((yNew + cyChild) > cyScreen)
    {
        yNew = cyScreen - cyChild;
    }
 
    return SetWindowPos(hwndChild,
                        NULL,
                        xNew, yNew,
                        0, 0,
                        SWP_NOSIZE | SWP_NOZORDER);
}