low level format
low level format means to overwrite the hard drive, making it difficult to read deleted material. Freeware would be killdisk. The free version overwrites with one single pass recording zero's to obscure anything left on the hard drive. Since zero's are predictable a skilled technician could filter out the zero's and still read what you thought was deleted. The upgrade version of killdisk, not free, has more secure features such as random digit overwriting, much more difficult to revive since randomness is entirely unpredictable. It also has the ability to do multiple overwrites, further concealing your erased information. You may be giving the computer to the kids today but it may end up elsewhere months or years later. If you have sensitive financial or copyright information i'd reccoment taking precautions now.