reboot and erase disk without admin passwd?

intel imac lost admin password

iMac Intel core 2 duo 20” OS10.5.5

I picked up this computer on an eBay auction. On receipt it works fine but it has no administrator password. I had not expected this to be a problem, as I was going to do an erase and install using system disks. However there is a problem. I cannot boot the computer up from disk using the c key, nor can I boot it in single user mode using cmd-s. Does this mean that the mac has had open firmware password protection activated, and if so is there any way I can install a new system without the admin password?

From what I understand, the firmware password exists precisely to prevent anyone from booting up the system from even an installation disk. Contact the seller and ask for the password or send the computer back if he doesn't have it.

Instead of holding c, press option. It's a second (and usually failsafe) way of booting for an installation disk.

Or connect to another Mac with iMac in Target mode - start and hold down T -then use disk utility to erase the HD

If a firmware password has been set, you can not use ordinary means to access the machine to make changes, and that includes target mode. Oneironaut's advice is spot on. If the previous owner can't give you the password, send the machine back.

Please read the following Apple KB article regarding firmware password usage.

Regards.

As has been pointed out by chscag and Oneironaut, to get into the Firmware Password Application to disable, guess what? You have to be able to get in and with no password no getting in. Have a look at this link:-

How To: Disable Mac OSX firmware password - FileCluster Reviews

I wonder if it could be stolen?

iMac Intel core 2 duo 20” OS10.5.5

I picked up the computer on an eBay auction. On receipt it works fine but I don’t know the administrator password. I had not expected this to be a problem, as I was going to do an erase and install using system disks. However there is a problem. I cannot boot the computer up from disk using the c key, nor can I boot it in single user mode using cmd-s. Does this mean that the mac has had open firmware password protection activated, and if so is there any way I can install a new system without the admin password?

Yes to oneironaut I agree my concern is that this could be a stolen machine - I will be returning as the seller has misrepresented it on the auction site.
Just out of interest how far would one have to go to disable the protection. for example were I too replace the hard disk with a new one?

The fact you cannot boot from an install disc very much indicates what has been stated above. Send it back period.