There is an example of a simple tree control on MSDN by Kyle Marsh written in API/C.

Once you dig into the code you will see that even though its functionaly simple, there's a huge amount going on and major sections rely on the richness of the Windows GDI. As a side note I have personally had to write a commercial quality treeview OCX and its a lot of work!

As a compromise it might be easier to do something with the grid control and turn it into a hierarchial version but it begs the question whats so problematic about redistributing the common control OCX?