|Do you want to restore it to factory with an operating system installed or do you want to wipe the drive completely and leave it a blank slate?|
Do you want/need to render any data completely non-recoverable?
There are basic tools readily available with which you can "wipe", partition and format a drive to give it a blank appearance however those tools usually do not erase any data, only wipe out the address tables that identify which sectors have been written and which are available for writing. There are any number of tools available to recover data from "formatted" drives.
There is a tool called Killdisk which is supposed to overwrite each sector with Zeros up to 7 times thereby rendering any previous data non-recoverable.
These tools are all run from a bootfloppy or bootable CD. The ultimate protection would be to take the drive out and dis-assemble it then take the platters from it and completely destroy them. High temperature incineration or shreading would suffice.
Goin' Fishin' (Some day)