Explicitly, from Install CD 1 of either Panther or Tiger you can, as 'root' or super-user after you have set up a password for it, apply new passwords of your own choosing to all existing accounts on the drive. While that may give you access to all of whatever is on the drives, nor should the owners concerned have let the iMac go without wiping its drive beforehand. Your best course is simply to buy and use your own copy of either 10.3 or 10.4, the distinction being only in the amount of RAM present (or augmented by you) and the CPU speed of the iMac. You will need your own install CDs to be legal and able to look after your iMac properly.

You could also, of course, simply erase/reformat the drive and install OS X and/or OS 9, and thereby remove all obstacles. Either way, you need OS CDs.

As bluekatt points out, a G3/500MHz Mac is able to run either above flavour of OS X with distinction, and Jaguar too, but that is a less rewarding OS.

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