dhtmlxWindows

Is the dhtmlxWindows component used inside the scheduler?

I need to popup a window from the lightbox and thought it might be possible to use that component if it is already part of the scheduler library.

No, it’s not a part of the library.
But you may check dhtmlxMessage, which is included into the scheduler

Docs:
docs.dhtmlx.com/doku.php?id=dhtmlxmessage:toc
Demos:
dhtmlx.github.com/message/

Thanks I will look at that.