Password panic!

Question

Hi there, I'll try and be as succinct as possible with this...
Recently purchased a new imac. I am selling my old machine that had Logic 8 installed. I bought the Logic 9 upgrade for my new machine and wanted to erase Logic 8 and it's large sample content from my old machine.
I followed instructions on how to do this from these very pages. I then went into system preferences to change the account name and password to that of the person who is buying my old imac.
I've successfully changed the account name, but am unable to change the password because it won't accept my old password. The hint is the same, I've been into keychain access and tries various other solutions. Because I'm still logged in to the computer it's fine to use, but obviously I can't install/download etc. I really need to sort this out to sell the computer.
I could re-boot from my 10.4.10 disc, but the computer is currently running 10.4.11, and it doesn't seem to like that much.
Should I just re-boot/re-install 10.4.10 and go from there. Ideally, I'd like to be able to change all administration and password privileges to the new owner.
Thanks heaps in advance for any advice. I'm a musician/sound designer, never really had to deal with problems of this nature before.
P.S Could I have erased something I shouldn't have when deleting the Logic content and preferences?

Answer 1

"I am selling my old machine"
You really need to wipe it clean to remove all of your personal information.
See Erasing disks securely
Restoring your computer’s software
Start up from your install disc, go to Disk Utility and select the disk and click erase - to securely erase data click Security Options and Erase Free Space which will entirely wipe your disk, overwriting it with zeros so that no data is recoverable.
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