I was just reading a site of their selling of older computer products and laptops. They said "meticulously cleaned" which got me thinking. What does this exactly mean?

I would think a simple re-installation of the OS would automatically clean out the harddrive. When I say this, I mean deleting ALL partitions when setting up the OS (namely Windows). So am I wrong to think this? What can I do as user do, to meticulously clean a laptop. (thinking about buying one from a friend)