You could start the computer from the installer DVD media disc and choose options in
the menu options, such as Utilities and other items; among them is a cue to reset the
password. In some vintage OS X installer discs, (when used to boot the computer) a
few other options appear in the drop-down menus that can be helpful.
You could start with the C key held, and keep holding it down for quite a time, while
waiting for the computer to try to run from the reduced OS X bits on the DVD. Or
you may choose to try the D key to see if any other volume of OS X can be found
on startup and choose (the DVD) that would do the correct job of allowing you to
change the password. Easiest with the install DVD, harder without it.
•Resetting a Forgotten (lost) Account Password - Mac OS X 10.6 & Before:
https://kb.wisc.edu/page.php?id=3853
Five ways to Reset a Lost admin Password: (see choice #3 for using DVD)
http://tidbits.com/article/14437
Another way without the install DVD, uses a command-line or single-user mode:
Snow Leopard 10.6 password reset, no disk method
This also is where one would be able to Installer a fully new OS X, so be careful to
choose the menu options and not the main installer, or disk utilities that can also be
used to ready a replacement hard drive or a second one for a new installation of OS.
Some keychain passwords and other items may need attention, so as to access
other functions that may have been set there. I try to not commit all my passwords
to the Keychain, and so seldom have need to use it.
Hopefully this helps somewhat.
Good luck & happy computing! 🙂