Disposing of an old Windows laptop? Here's the safest way to erase your personal data (for free!)
Briefly

Whichever course of action you take, though, your most important task is to permanently delete all your personal files from that PC before you pass it along.
For laptops, the simplest route is to reset the PC, choosing the option to remove personal files and reinstall Windows. Make sure you choose the Remove Everything option.
Read at ZDNET
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