If you are not un UNIX and running "du" command, you need a software to analyze your hard disk and see which are bigger file on you computer and smaller. I will show you two free tools who make your work easer for this argument.
The first one is TreeSize Free.
TreeSize Free tells you where precious space has gone to. TreeSize Free can be started from the context menu of a folder or drive and shows you the size of this folder, including its subfolders. You can expand this folder in Explorer-like style and you will see the size of every subfolder. Scanning is done in a thread, so you can already see results while TreeSize Free is working. The space, which is wasted by the file system, can be displayed and the results can be printed in a report. TreeSize Free is freeware for Windows
The other one is a small also free applications for windows called disksum.
Sample output from running it in my C:\WINDOWS:
