Concidering that most peoples don't have the knowlege nor the tools needed to recover ereased data, normaly, you don't need to do anything fancy.
Then, those who usualy buy used computers are often casual users with prety limited computer knowlege who normaly can't afford to get the tools needed to recover data from a reformated drive...
Most of the time, simply reformating and reinstalling the operating system is enough. Defragmenting after the reinstallation is a good idea as it tends to scramble whatever may be left in the free space.
This will effectively make any personal data totaly unrecoverable for over 99.999% of peoples.
If you want to me more thourough, create a new administrative profile, suppress all other profiles. When offered if you want to preserve files assodiated with the old profiles, sellect DON'T preserve files. Defragment.
Delete about half of the files left.
Defragment the drives.
You may optionaly copy a bundle of inocuitous and common files, like the fonts folder and some other folders under the windows folder. You can also add some videos, music and pictures to the lot. Continue untill you get a disk full error... Delete about half of those.
Delete the remaining user files and defragment again.
Reformat and reinstall the operating system. Defragment.
This will effectively nuke your data for the above mentioned peoples, make it almost impossible for the casual hacker and make it excessively hard and costly to recover for the rest.
If there are more than a single drive, deleting everything on the non-OS drives and emptying the recicle bin is probably enough, but you can always reformat.
The point is not to make your data absolutely irrecoverable, but make it so that it takes to much efforts and cost to much to recover them compared to the potential worth.
Electro, April 2012
