It's something beyond just securely deleting files.You obviously know that when you delete files and other stuff without shredding, their content still remain on your computer and some program can recover these files.
This is happening because when Windows deletes a file, it does not actually clean the file contents. Instead, it just declares the space that was taken by the file as empty. This makes file deletion very fast but is a real problem when the deleted file had a private content. But you probably know this, I just babbling here....
So, I use this tool for overwriting.I've tested both Eraser and CCleaner but this tool has the fastest overwriting I've ever tested. Hope it helps you:
http://www.mil-shield.com