You may be able to reset the password and other things once you access the system via install DVD... This includes making a new account. If you can 'restart the setup assistant' as suggested in second link below, that may be a key to creating a new account, an Admin. Later, you could delete the older one. But see this first:
•OS X: Change or reset an account password (snow leopard and earlier)
With a powerPC Mac, the newest OS X versions (10.6 & later) are not supported, so there is no Recovery disc partition or other stuff, and as such, the instructions you may find will vary across the range of change. Note there is a way to reset the password without an install DVD, some suggestions will not work on a powerPC Mac. Look through suggested methods until you see the one(s) that work, ie: two here use commands...
•Resetting an OS X Password without having Installation Disc:
If there is an Open Firmware password, that requires a few different things to get past.
Good luck & happy computing!