Wiping free disk space
When you delete a file, Windows removes the reference to that file, but doesn't delete the actual data that made up the file on your hard drive. Over time, this data will be overwritten as Windows writes new files to that area of the drive.
This means that, given the right software, someone could reconstruct all, or parts of files that you've deleted. For privacy and security reasons, you can set CCleaner to wipe the free areas of your hard disk so that deleted files can never be recovered.
Note: Wiping free space can take a substantial amount of time.
Method 1 (Automatically wipe when cleaning):
Method 2 (Manually wipe with Drive Wiper):
Changing CCleaner settings
To select the drives whose free space CCleaner will wipe, see this topic.
Wipe Free Disk Space limitations
CCleaner can't wipe every deleted file from your free disk space. There are some limitations, because of the way Windows stores some files. Here are some examples: