RECT dlgRect = {0};
GetWindowRect(hWnd, &dlgRect);
int widthResolution = GetSystemMetrics(SM_CXSCREEN);
int heightResolution = GetSystemMetrics(SM_CYSCREEN);
int newXPos = (widthResolution / 2) - (abs(dlgRect.right - dlgRect.left) / 2),
    newYPos = (heightResolution / 2) - (abs(dlgRect.bottom - dlgRect.top) / 2);
SetWindowPos(hWnd, NULL, newXPos, newYPos, 0, 0, SWP_NOSIZE | SWP_NOZORDER);