Delete Files Permanently in Windows

Whenever you delete a file, Windows will remove the index for the file and prevent the OS from accessing the file’s contents. However, hackers with physical access to the storage media can  recover the file’s contents until it has been overwritten by another file—which might never happen. Similarly, files that have been EFS-encrypted leave behind the unencrypted contents of the file on the disk.

However, with the free SDelete tool, you can overwrite the contents  on your disk to prevent deleted or encrypted files from being recovered.

To use SDelete to overwrite deleted files on drive C, run the following command:

sdelete -z C:
SDelete - Secure Delete v1.51
Copyright (C) 1999-2005 Mark Russinovich
Sysinternals - www.sysinternals.com
SDelete is set for 1 pass.
Free space cleaned on C:

From Microsoft Technet