It's not just computers. There was a show on last year about companies that take old copiers in trade for new ones. Most all of them have hard drives, and it was unbelievable what they were able to download from these traded in copiers. Most everything that was ever copied was able to be recovered and reprinted.
Don't buy into all of this, "Reformat with all zeros", stuff. There are many people in jail because police were able to recover damaging evidence from hard drives people thought were "erased". There is only one way to erase a hard drive, and that is on a sidewalk with a hammer, or else as sledgemeister has demonstrated. Bill T.