Valid Window?

Hi, I'm using the SetParent API to force another app to be a child of mine (set into a picturebox). Easy so far, but if I later use IsWindow with the same handle, it fails as if it's no longer recognizing the window (because I made it a child of mine??). Is it no longer a valid window? I want to grab menus off of it, but without being recognized as a window, APIs like GetMenu fail. What can I do?

Thanks in advance