Data is not permanently deleted when you simply delete data in microsoft windows. It still remains on the hard drive but Deallocated. So the deallocated space should be overwritten to make it harder to be recovered.
Microsoft windows little-known command line tool called Cipher.exe which used by administrators to encrypt and decrypt data on drives have a switch to prevent authorized recovery of deallocated data.
How to Use the Cipher Security Tool to Overwrite Deleted Data
The cipher /w command does not work for files that are smaller than 1 KB. Therefore, make sure that you check the file size to confirm whether is smaller than 1 KB. This issue is scheduled to be fixed in longhorn.
To overwrite deleted data on a volume by using Cipher.exe, use the /w switch with the cipher command:
1. Quit all programs.
2. Click Start, click Run, type cmd, and then press ENTER.
3. Type cipher /w:folder, and then press ENTER, where folder is any folder in the volume that you want to clean. For example, the cipher /w:c:\test command causes all deallocated space on drive C to be overwritten. If C:\folder is a Mount Point or points to a folder on another volume, all deallocated space on that volume will be cleaned.
Data that is not allocated to files or folders is overwritten. This permanently removes the data. This can take a long time if you are overwriting a large amount of space.