As with any freeware, be sure and do a custom installation otherwise you might get things installed you do not want.

To get started, download and install
WinDirStat. Go with the recommended installation.
Select the drive you want to evaluate, or "all drives" if you have multiple hard disks. Click "OK".

Depending on how big your hard drive is, WinDirStat will take about 5-10 minutes to evaluate your drive.

When the scan is complete, you'll be presented with the summary screen. The top half lists files and folders sorted by file size. If you click on a folder or file, the corresponding color block will be highlighted below.

Alternatively, click the large blocks of color to find out which files are taking the most space.

Once you've identified the culprit, right-click the file and choose "Delete (to recycle bin)" for later disposal, or "Delete (no way to undelete)" for permanent, irreversible deletion.