Clearly, if you’re going to be selling your computer to a stranger (or anyone else), the last thing you want is to leave any data on it.
So, My Disk Wiper is a good program, and will do it…even asks you twice, “Are you sure” type questions. It can be a long process, though.
gHacks recommends checking the effectiveness with Recuva or Testdisk…I think that’s a good idea no matter which software/method you use to erase the data.
I’d recommend bookmarking this link…no point in downloading something you won’t need anytime soon: Waste of space and it might be updated/improved or a better utility might come out in the meantime.
you can find it here: http://www.myportablesoftware.com/mydiskwiper.aspx and/or read more about it here: http://www.ghacks.net/2016/02/01/my-disk-wiper/?_m=3n%2e0038%2e1817%2ehj0ao01hy5%2e1w3p