You have to overwrite them. What happens when you delete something is it's marked to be overwritten the next time you write to the hard drive. It's not really deleted, just written over the next time you write to the disk. The system sees the marked space and just adds it into the free space on the drive. Thats why if you accidentally delete something, it's very important not to write to the disk again until you recover the data. I've seen this happen with a compact flash memory card, I couldn't recover all the pictures that were deleted because the person took another picture with the camera and the cf card.
There is no such thing as shadow memory.