GetFoldersize is a handy tool that will quickly let you know exactly which folders are using most of your hard drive space.
The process starts by scanning a particular drive. This may take a while, as by definition GetFoldersize has to examine every file in every folder, so you'll need to be patient.
Once the scan is complete, though, you'll immediately see a list of folders sorted by the amount of drive space they're consuming: \Program Files, \Windows, \ProgramData, whatever it happens to be on your system.
Sometimes you'll want to explore this further. The \Program Files folder on our test PC contained 25GB of data, for instance, but which applications were the worst offenders? Double-clicking any folder of interest allows you to zoom in and see the folders it contains, also sorted by size, so the resource hoggers are immediately highlighted.
And if, at any point, you spot some files you don't need, then it's easy to select and delete them in the Files pane, so freeing up some valuable hard drive real estate. Don't wipe anything unless you're entirely positive that it's surplus to requirements, though - make a mistake here, delete something important, and you could cripple your PC. If in any doubt at all, just leave it alone.
This is the portable version of GetFolderSize.
An effective hard drive space visualisation tool that can quickly reveal the most space-hogging folders