It’s well known that when you delete files and folders in Windows they are not technically deleted. When you delete a file, the disk space used by these files is tagged as available for use. This allows the files to be reconstituted using various free recovery utilities such as SoftPerfect’s File Recovery or Piriform’s Recuva. The blocks must be overwritten to actually eliminate them completely.
Windows has a native utility named cipher.exe that can wipe those pointers and make sure the data is actually purged. Cipher.exe can overwrite all free space on your disk thus insuring files you have deleted and actually gone.
This is a safe utility. I have run this command many times over the years. You can also setup a scheduled task and run weekly to keep your systems clean. Launch a command prompt as administrator (right-click cmd.exe and select Run as administrator) and type the following:
c:\cipher /w:X where X is the drive letter you want to clean.
You can run this on your c:\ drive without any issues. Also note that the larger your drive, the longer this will take. For reference, a 1TB drive 3/4’s full took about 3 hours.
Example (this is on Windows 7):
C:\Windows\system32> cipher /w:c
To remove as much data as possible, please close all other applications while
running CIPHER /W.
Writing 0x00
………………………………………………………………………………………………………….
Writing 0xFF
…………………………………………………………………………………………………………..
Writing Random Numbers
…………………………………………………………………………………………………………..
C:\Windows\system32>
Further reading on cipher.exe options is available here:
http://technet.microsoft.com/en-us/library/cc771346(v=ws.10).aspx