iMac - DK administrator password on eBay aquired computer

Hey Kippdog and welcome to Apple Discussions,

Tried reinstalling OS from the disk-holding down the C key, but I'm asked for the password to allow reinstall. How do I do a clean install to set myself as administrator?

Well it sounds like this iMac was on a school network, so the best approach is to erase the hard drive and do a reinstall.
You'll have to use target mode and another computer to do this:
http://docs.info.apple.com/article.html?artnum=58583
Use disk utility.
Then do a regular install and set yourself up as administrator.

Richard

Change Password

Mac OS X:

Changing or resetting an account password via GUI:
http://docs.info.apple.com/article.html?artnum=106156

Changing password from single user mode:
You can also change the administrator's password from single user mode or create a new administrator account.

You need to get into single use mode for steps one and two that are listed below.
This page will tell you how to get into single user mode.
http://www.securemac.com/macosxsingleuser.php

Basically, you hold down the command-s key when powering on. The command key has a little apple symbol on the lower left. It is between the alt/option key and the space bar. On a PC keyboard, it will be the windows key, I think.

1) You can change the password on an account. ( Do you know Unix. You are in a Unix single user console. )

Follow the instructions on screen for accessing the startup disk in read/write mode which are:
/sbin/fsck -y
/sbin/mount -uaw

To find out the users on the system type:
ls /Users

One of these accounts will be the administrator.

Pick one of the users and type:
passwd a-user-name
and enter the new user password. You need six characters.

The root account isn't enabled by default. I am not sure if changing the password on root will enable it.

2) Get the Mac to set up an additional administrative account. You can then change the password on your old account.

This was Posted by: JoseAranda at September 9, 2006 3:48 AM

OK, restart your computer, hold down Command-s and type in the following:
/sbin/fsck -y [Enter]
/sbin/mount -uaw [Enter]
rm /var/db/.applesetupdone [Enter]
# The rm command is the remove command which deletes the file.
# Robert: I'd rename the file via: mv /var/db/.applesetupdone /var/db/.applesetupdone.old
reboot [Enter]

Once you've done that the computer reboots and it's like the first time you used the machine. Your old accounts are all safe. From there you just change all other account passwords in the account preferences!!
end of posted by: JoseAranda

http://www.askdavetaylor.com/howdo_i_reset_my_mac_os_x_admin_rootpassword.html
You will need to scroll down to see this post. Search for applesetupdone

Once you have a new administrative account, you can change the password of your old administrative account
blue apple > System Preferences > Accounts

Give administrative privileges to an id
http://discussions.apple.com/thread.jspa?messageID=607379

How to crack Mac OS X password
I recommend changing your password, but with a lot of computer work you can find out your current password.
s http://www.wap.org/journal/crackmacpasswords.html