It is a common knowledge now that simply deleting files from a disk drive does not actually delete their contents, it just marks them unavailable in the file system. The actual contents of these deleted files still persist unless they are overwritten by some other files. This is why people are advised to securely erase their sensitive or personal files that could contain private data, for example, your tax return files, the scanned copies of your passport and so on. For this, you can use many free tools including Piriform CCleaner.
The popular disk cleaning program CCleaner can also securely erase your hard disk drives. It can perform the secure erasing of only the free space available on your hard disk drive or the entire disk drive itself. For this, you can launch CCleaner from its desktop shortcut and select the Tools section followed by Drive Wiper.
Here you are given options to wipe various partitions of all kinds of disk drives attached to your system. You can choose to wipe only the free space available on the disk partitions or the entire disk drive. You can also select the secure erase method from simple overwrite (1 pass), advanced overwrite (3 passes), complex overwrite (7 passes), very complex overwrite (35 passes).
Next you can select the disk drive partitions that you want to securely erase and click on the Wipe button. This will initiate the secure disk erasing routines resulting in removing the entries from the file table of the file system. The process is started without any confirmation if you are just wiping the free space. But if you decide to erase the complete disk drive partition, then you have to first type in ERASE in the text box before proceeding. This prevents you from accidentally erasing the contents of any of the hard disk partitions.
Conclusion: CCleaner is not just a system cleaning tool anymore. It can do many more things like securely erasing the contents of your storage disk drives. It can secure erase both the free space on disks as well as the full disk partitions.
You can download CCleaner from http://www.piriform.com/.