How to update Mac OS X without admin. password?

I bought a power mac g4 at a fleamarket, and need to update the software, but I don't have the administrator password. The administrator was logged on when i first started it up, but there is little I can do, and was wondering if there was a way to get past the password or reset it. I tried the keychain access... show more I bought a power mac g4 at a fleamarket, and need to update the software, but I don't have the administrator password. The administrator was logged on when i first started it up, but there is little I can do, and was wondering if there was a way to get past the password or reset it. I tried the keychain access but that is locked and does not show anything.
Update: i do not have the install disk
Update 2: Mac os x 10.4
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