I think the only 100% way is to replace the HDD. As that is not practical the next step is to re-install the operating system. This erases and reformats the HDD (but information could still be recovered from it by a specialist). If you are certain the recipient is not going to take it directly to the specialist then that's your best bet. Even this may not be an option if you don't have your installation discs and are not sure what to do (someone could do it for you - a pal perhaps). The only other way is to remove stuff the best way you can - running as many "cleanup" utilities to finalise.