Hello and Welcome to Tom's Hardware!
The tree view of WinDirStat should show you which folder has content filling up your HDD.
Possibly it's hidden not permitting you to see the folders, as the hidden AppData folder. Might click on Windows Explorer, tools, Folder Options, View, and change the radio button to 'show hidden files, folders, directories' Then take another WinDirStat look. When done, change it back
Occasionally a virus will begin filling up your HDD, so run a full scan with a good Antivirus software.
Sometimes the Recycle Bin will fill up with 'deleted' files that you haven't cleaned out. Also the Temporary Internet Files or Temp or Tmp files may be filling up, if you haven't cleaned them out recently
If you are running Win-7, click on Computer, then right click on your HDD, properties, then click on the Disk CleanUp button. It will search your HDD for disposable files, and give you a choice of what to 'clean up''. Choose the Recycle Bin, Temporary Internet Files, and Temporary Files, then OK
See if that cleaned up you HDD.
Lastly, if you are running the Norton Utillities, or similar applet, a 'hidden' recycle bin can store all the deleted files you 'deleted' from the regular recycle bin. In that applet or in the Recycle Bin, check empty hidden recycle bin, if appropriate.
Hopefully that will get you a squeeky clean HDD.