I am giving away one of my laptops to a friend of mine. It has Windows Vista operating system. In good condition but I do not need it (as I have replaced it by an ultra-powerful Alienware gaming laptop!). The other person is of course trustworthy but just to be safe I want to get rid of personal files from my computer. I have some straight and gay porn ( I am bi ), I have some games, I have some personal information. I can simply delete all of that. But I know that it is not really gone from the computer as it is still hidden in the harddisk in some way. How can I delete files completely while preserving the operating system so that he can use the computer just as it was before?