I have a window to be positioned on top pf another window.

I have the following code:
if (pWnd == NULL)
{
pWnd = FindWindow(NULL, L"Menu");
}

if (pWnd )
{
CRect lpR;
pWnd->GetWindowRect(&lpR);
SetWindowPos(&CWnd::wndTopMost,lpR.left,lpR.top+20,0,0,SWP_NOSIZE);
}

But it makes the window on top always(of all windows).

I just need to make it on top of a particular window(if it is visible/on foreground).

please help me regarding this