When you delete a file from the recycle bin in Windows, rather than deleting the actual file, the space the file is occupying is simply marked as free so that another file can come and use those blocks on your hard drive. One way of making sure that your data is not recoverable is to simply overwrite all the free space with random data.
When the command prompt opens type:
cipher /w:F:\
Where F is the letter of the drive you want to securely overwrite the free space on.
Alternatively, if you want to make use of a third-party tool you can use SDelete from Microsoft’s Windows SysInternals suite of free utilities. Simply download the file and extract it.
Please open “cmd” and now run the following:
SDelete –c F:
Where F is the letter of the drive you want to overwrite the free space on.
This will work on both Windows 7 and Windows 8