To ensure clean hd before selling unit: Best practice?

Hello, All
I'm planning to sell a laptop that's been in use for a couple of
years, & want to ensure that after the new owner gets the unit there's
zero chance of her/him coming across old/sensitive data.
Is a total re-format of the hard drive and a re-install of XP
sufficient? Or is it necessary to first use a drive-wipe utility like
DBan or Eraser?
Thanks in advance [:-)
riprap.

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