Download a copy of the ISO for Ultimate Boot CD (UBCD) from
Ultimate Boot CD - Overview.
Burn a copy of the ISO to CD-ROM.
Boot from the CD-ROM and run one of the hard disk wiping tools on the UBCD, it has a choice of:
- Darik's Boot and Nuke 1.0.7
- CopyWipe 1.14
- Active@ KillDisk Free Edition 4.1 Free Edition. Supports "One Pass Zeros" method only.
- PC INSPECTOR e-maxx 0.95 Actual version is 0.95 Build 775, but is referred to as 1.0 on product website.
- HDDErase 3.1
Keep the disk as it's useful for other things.
Reinstall the operating system onto the laptop or leave it to the new owner to do it. They may want to do a fresh install themselves anyway, I know I would.