Resizing controls...?

I have a little problem with a form. I have in it two controls, a treeview showing some tasks, and a textbox showing the content of each task.
I need to resize (with the mouse) the limits between the two controls, because one may be greater than the other. How can I do it??????
I think about it in some like the explorer of Windows 9.x, or the MSOutlook, do you understand?
I suppose there is an OCX that can do this, or perharps I must do it with code?
Thanks in advance, and excuse me for my bad english...

You could achieve this in code although it would be much easier with an appropriate third party control.

I use the Sheridan (now Infragistics) splitter control.

This control is a container that can be easily split into as many panes as required. This allows the user to resize the panes (which in turn automatically resize the contents of those panes).

I don't know if you can buy that control individually though, they probably bundle it with a few others.