From techrepublic.com
To make sure a deleted file can’t be recovered, you’ll need to use a third-party shredding tool. Here’s a look at three such free programs: Eraser, File Shredder, and Freeraser.
We all know that deleting a file in Windows doesn’t actually remove it from your hard drive—that file may be stored in the Recycle Bin, which keeps it intact in case you wish to restore it. Even if you bypass or empty the Recycle Bin, the file’s clusters still reside on the disk unless they’re overwritten with other data. And even then, a professional recovery product can often restore an overwritten file.
You may want to completely and securely delete a file if it contains sensitive information, or if you plan to dispose of your computer and want to be sure no one can access or view the file. You can turn to a third-party deletion or shredding tool that fully deletes a file by overwriting it multiple times with random data so the file is virtually impossible to recover.