Are you planning to sell your computer or hard disk, or just throw it away, and was it holding highly classified and confidential data? Do you know when you are deleting files on your hard disk, they are not completely and permanently deleted and anybody can access them? When you delete a file the space is simply freed for another file and the file is never deleted or overwritten unless the space is used by another file, and this may never happen, disks having so much space these days.
Even if you format a disk before selling it or throwing it away your information can be retrieved. In order to completely erase, delete information there are many utilities, some free and some commercial, available on the Internet. Some of them are discussed in this LifeHacker blog post.
