I use Secure Shredder a built-in tool in
Spybot - Search & Destroy
The Secure Shredder will permanently remove files you won't be able to recover files after using Secure Shredder.
You can also try other freeware's like:
Simple File Shredder : Simple File Shredder is a utility that securely deletes your files so they cant be restored using recovery software.
Tiny Shredder : Tiny Shredder is a small file utility that will completely erase the contents of sensitive files and folders that you specify.