If it's the only drive in the laptop, stick the DBAN cd in the laptop, boot it, and it'll clear everything off it. Inlcuding the OS. It'll be a completely virginal disk.
I would mention tho, I've experienced DBAN not liking to be booted in a laptop - if this is the case, many laptops (HP's in particular) will have a "Disk Sanitizer" in the bios. This will do exactly the same job. Total erasure of evidence.