Does every windows which shows itself while working on any Windows O.S ( lets say Windows 2k) a registered one? - Programmer

Hello All,
I am learning windows programming and using spy++ to view the class
names and other properties of the windows for better understanding.
It works fine for most of the windows that show themselves.But when
windows generated by java program appear on the surface,it will only
recognise the parent window with some classname like java.awt. but
won't recognize the child windows like buttons,dropdowns inside the
main java window.
Earlier I was thinking that every window which we see in WindowsO.S
shoule register itself with the O.S?But here the child windows of java
haven't registered themselves and then also we are not only able to see
them but also interact with them.
I am confused now....
Does every windows which shows itself while working on any Windows O.S
( lets say Windows 2k) a registered one?


Thanks and regards,
Yogesh Joshi