DirSize is a program to explore and monitor your hard drive. It displays a tree structure regrouping all the folders according to their types (files, images, music, etc.) and their respective sizes. You can browse the file tree to see the details of the included files. Besides, if you want to know the details of a specific folder, simply select it and click on "Open in Explorer".
DirSize is free software and once installed, it immediately starts to scan your disk (indexation of all files and folders).