iBook ownership

Couple of years ago I decided to upgrade from my Powerbook Wallstreet. I bought a used iBook and the previous owner gave it to me with no password set. I changed the password to suit me and loaded my software. no problems. Then, at some point I needed to download some new software and up came the name of the previous owner. I finished the download and went looking for where to change her tag to my own. Went and looked everywhere and thought I'd done it. Last week tried to download neooffice and there it was again her name. She's long gone and I'm getting upset with not being able to flush out her ID and introduce my own. How do I do this???

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