There are good paid privacy programs that can securely delete and wipe the free disk space e.g. SecureDelete in

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Undelete 2009, Privacy Eraser, Webroot Window Washer, Drive Cleanser in
Acronis True Image but there are users who prefer to use free tools in wiping disk space. Below are the two free utilities that include wiping of free disk space:
- CCleaner – this popular tool will not only clean-up for temporary files in Windows but there is an option to securely delete (using many passes) the files and to wipe the free disk space
. It’s also portable and recommended in several community forums. - Eraser – another free tool to remove sensitive data by shredding and wiping the free disk space in Windows
Note that at the time of this writing the free tools to clean-up the system such as System Cleaner by Comodo, FCleaner and ATF-Cleaner do not have the option to wipe the free disk space. Also, the File Shredder in Spybot - Search & Destroy does not wipe the free disk space.
Windows and other operating systems have built-in disk cleanup but it is lacking the features of overwriting the deleted and free disk space.
Tips: Schedule a system defrag after the secure deletion and wiping the free disk space. This will make your clean-up more effective in addition to keeping your hard-drive healthy.