Boot from the old OS DVD, and after the language selection, go to the Utility menu and select the password reset option. I forget the specific name, but it should be obvious enough. That should allow you to "reset" the password without needing to know the old one.
When I first got this Mac, somehow I neglected to create a password. When asked for one, I just click and all is well. Now I want to install Lion and it really, really wants a password. Clicking doesn't cut it. To create a NEW password, I have to give them my OLD one - Catch 22.
Any thoughts?

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