Help with popup... PLEASE!!

I need a popup control or script that I will be able to place another control in and have the response sent back to a textbox on my asp.net page.

Here is the best I can say in plain english. I will have a button on my page (.aspx) that needs to open a popup with a treeview in it. Then have the selected item from the treeview populate a textbox on the page.

I assume the popup will need to be done via javascript, but I'm not sure how to get the contents back to the textbox.

Well, you would need an onclick event in the treeview, that would change the address of the original window (i.e. use GET to submit the selected value back to itself), or at least POST a form back to itself.