When I bought my Mac mini, I got it without the installation disk and I also didn't get the administrator password. Is there a way to find out in my computer the admin. Password and get a system install disk for it?
If the machine doesn't have an Open Firmware Password set, you can reset the password with single-user mode (if it does, you won't be able to access single-user mode without entering the Open Firmware password).
(Source for the instructions)