The only 100% sure way to keep unwanted files out of the hands of others, is to not sell the unit with the HDD.
Short of that, a simple format before running the recovery CD should stop probably 95% of people from accessing anything on there. The remaining 5% could probably get it no matter what you did if they cared enough.
Just look for some kind of boot disk or CD that has a zero filling format program in it, and run that. It's not 100%, but it's going to be pretty damn good. There should be a few free ones out there if you look.
I have a Toshiba Laptop, it came with a recovery CD. By using a recovery CD does it remove all and restore to the day it was bought? Is there anyway traces could be left behind? I am selling my laptop and want to be sure nothing could be recovered.
Thank you in advance.