I am selling my old computer to a used computer shop, and i wanted to know how to delete all of the files that i ‘deleted’ after putting them in the trashcan.
A friend of mine, who works with computers but who is on holiday rightnow, told me that when you delete files in the trashcan they aren’t completely deleted and can be seen somewhere else on my computer.
The reason i need to do this is for obvious reasons: security and privacy. Those being private excel sheets and the like.
Can anyone point me in the right direction? I’m kind of afraid i’ll end up destroying my hard-drive…but i know someone here will give me full-proof instructions 
Johnny
