What is the safest way

Question

What is the safest way to erase a hard disk in order to re-install OS afresh? Laptop is a Fujitsu Siemens Esprimo Mobile V5535. Contacted Fujitsu and they gave me a link to a part of their website with a program to erase the harddisk. Tried link and it doesn't work. Tried to call Fujitsu again but office closed until next week Tuesday (so much for customer support!). Many thanks.