When you are ready to retire your old laptop you need to be sure you remove any critical personal data from it's hard drive. You should remove all passwords, financial data, e-mail messages and personal data. You never know where the hard drive will wind up and who might look at your valuable personal information.

The U.S. Department of Defense specification for sanitization of hard drives is DoD 5220-22M. You make multiple passes: the first pass inscribes ONEs, the next pass inscribes ZEROes, the third, fourth or more iterations write the government-designated code “246” across the drive. The goverment may actually run as many 8 passes, then inspect the drive with a Read-Verify review.

Overwriting data once is not usually good enough to prevent data recovery, it is recommended that a minimum of three passes are made so no data can be recovered.

Free programs that would sanitize your hard drive are: Darik's Boog and Nuke and Active Kill Disk.