with your name, using the System Preferences/Accounts Pane, and give yourself the right to Administer the machine.
Set the machine to auto logon to your name and all should be well.
You will need the administrators password to do this. I am assuming that the previous owner told you what it was. You cannot install software or make major changes without this password.
The other way, and the most drastic, it to wipe the drive clean and start again under your name.
Let me know if you have this password.
P
i've just bought a 2nd hand apple iBook G4 (running panther) and wanted to change the username (the name that the book is installed to....(like next 2 the house when in the Finder window))...PLEASE how do i change this to my name??

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