Tiger - remove old admin

Hi. About 18 months ago I bought a used iBook G4 14" laptop with Tiger installed and on "Replacement DVD" media with the intention of giving to my granddaughter when she was old enough to use it. She's now 8-1/2, so I decided to make it a Christmas gift. I knew that the previous owner's profile was still on it - the user privileges were not password protected, and I figured I could reinstall Tiger from DVD and start clean just before I gave it to her (now). That apparently was naive. Reinstall requires sysadmin privileges, and the sysadmin profile from previous owner _is_ password protected. I can no longer contact that person. What are my options to do a clean install of Tiger from DVD on this computer? Is there some relatively simple work-around to remove that admin profile and/or reformat the drive? Or am I condemned to physically replacing the HDD with a new one to do this (I have a suitable drive, I just didn't want to do what looks like a byzantine replacement operation unless there was no other choice. Thanks in advance!

-Sam