SpaceWatch for Windows

At the top level it will display list of detected top-level drives or root folders, along with the estimated percentage of total disk space used on that drive / root folder. For any other directory, it will show a breakdown that shows the rough percentages* of total used space that each file and sub-folder takes up; this gets more accurate as the program digs deeper of its own accord to get more information.

The program scans everything automatically; when you click to dig down a level or go back up one level, it prioritises that particular folder; from there it will dig right to the bottom of the tree, then start on the next branch, and so on.

(* For ease of viewing, the bottom 1% of files and folders are not shown in the pie, and files or folders taking up most of the space are capped to about 90% of the pie.)