The answer to this question is unsettling.
When you delete a file by putting it in your computer's Trash Can or Recycle Bin, it only removes the physical "address" of the file on the hard disk. The data still exist.
According to the security standards of the U.S. Defense Department, information from dumped computers is unrecoverable only after it has been blanketed with garbage files seven times. However, other research reveal that only by covering files 36 times is data restoration fully impossible.
In other words the most effective method is breaking the hard disk into pieces or melting it down.
For those who want to hide the truth or for others who want to protect themselves from financial exploitation there can be few bigger headaches than computers.
Modified from Be Careful How You Delete Your Incriminating Files