When I need to find why a hard drive is running out of free space, simply searching for files by size using Windows' built-in search capabilities isn't always up to the task. I can certainly find the largest files on a drive, but they lack the context of surrounding files and file types. Furthermore, you can't identify problems caused by large numbers of smaller files. Instead, I prefer to use the free WinDirStat directory statistics tool. Inspired by the KDE tool KDirStat, WinDirStat is a portable application that provides multiple views of file-space usage. It's compatible with Windows 95 and later.
The multiple usage views, as seen in Figure 1, simplify getting a quick overview of disk usage.
Figure 1: Get a quick overview of disk usage with WinDirStat (click to enlarge)
The expandable folder listing shows folder usage sorted by size, including percentage of space used, absolute size, and number of items. The extension listing in the top right provides a breakdown on usage by extension and provides a color key for the file types that are displayed in the Treemap view at the bottom.
The Treemap view is what I use to quickly get a usage overview. Blocks, proportional to size, represent files and are arranged so that each larger rectangular grouping is a directory or subdirectory. With a few seconds of inspection in WinDirStat, I usually know why a drive is getting full.
One caveat to keep in mind is that on Windows 7 and Windows Vista, WinDirStat's uninstaller doesn't correctly remove the All Users\Start Menu\Programs folder for WinDirStat or the installing user's desktop shortcut. You'll have to remove these leftover pieces yourself. However, WinDirStat doesn't actually need the installation routine, so you can just install it once and copy WinDirStat.exe and WinDirStat.chm to a network location or USB drive for mobile use.