The above article is good, but the steps are limited to regenerating the Setup Assistant. The steps in the comments for deleting the user account are for Tiger (10.4.x) and do not work on Leopard (10.5.x), since Apple deprecated the NetInfo db.

Most people will never have to do this, but if you are selling/giving away your old machine (or like me, frequently set up new machines for new users), these are handy steps to create a nice, fresh out of the box new user experience, while still having updated and installed all the third party software requested.

Note: These steps assume that you have already installed the operating system and third party software, and run any software updates. These steps will not only reset the Setup Assistant, but will also delete the user accounts for any users you specify. This will, obviously, erase any data in those user accounts. Do not proceed unless you understand and want this.

First, restart in Single User mode and enable changes to the system.

1. Restart the computer holding down the Command and S keys during startup.
2. At the prompt type the following, then press enter/return:

mount -uw /

Next, reset the Setup Assistant.

3. Type the following, then press enter:

rm /private/var/db/.AppleSetupDone

Now, remove the user accounts from Directory Services.

4. Type the following, then press enter:

launchctl load /System/Library/LaunchDaemons/com.apple.DirectoryServices.plist

5. Type the following, then press enter:

ls /Users

6. Note the short name (username) of the user account(s). In these following steps we will be deleting all the users, one at a time, from Directory Services.
7. Type the following, then press enter (substitute the short name above for the username below):

dscl . -delete /Users/<username>

8. Repeat the above step for each user account.

Finally, delete the home folders for each user account

9. Type the following, then press enter:

rm -rf /Users/<username>

10. Repeat the above step for each user account.
11. Type the following, then press enter:

shutdown -h now

Mac OS X inherited it’s file-level permissions scheme from it’s UNIX ancestors. The advantages of this approach are many fold, not limited to making it very difficult for malicious software to be installed on your system without your direct knowledge and consent.

But it can be difficult to understand this system, and thus I wrote this explanation of the three levels of access on a Mac OS X system. You can think of these levels as:

1. User
2. Admin
3. Root (aka System)

Users have free read/write access to pretty much anything within their Home folder (that’s the folder with your user name and the house icon inside /Users), and will likely never be prompted to enter a password for any changes made there.

Admin users have the same access to their Home folder, plus read/write to the contents of /Applications and most of /Library, as well as a couple parts of /System. They are not prompted for enter a password for changes to these locations.

Root has full access to the entire system. Root accounts are disabled by default and are recommended to stay disabled unless you have a specific reason to enable them and sincerely know what you are doing.

What this means in a practical sense is that if you are using a restricted (aka “Standard” or “Managed”) user account you can only screw up the contents of your own Home folder. While your data or documents might be affected, you are not going to inadvertently install something that impacts the system as a whole. If you attempt to install something that affects the areas outside of your dominion you will be prompted to authenticate with an Administrator’s user name and password.

Alternately, when you are using an Admin account you could be fooled into installing something that affects the system as a whole — without ever being prompted for a password — if the destinations for the installed files are limited to areas that Admins normally have read & write access. If you try to install something that modifies files owned by Root/System, you will still be prompted for your password (unfortunately, you can still do serious damage to the OS as a whole without ever touching Root-owned resources).

Entering an Admin user name and password allows you to temporarily assume the permissions level of the Root user (meaning you get access to just about everything on the system). If you are unsure of where the application asking for authentication is coming from or why it’s asking for permissions it would be safest to decline until you can get clarification. The vast majority of software on the Macintosh OS shouldn’t need administrator rights to run.

Also, by default, the individual users on a system — whether they are Admin or Restricted — do not have access to read or write each other’s files. If you create a new user, this new account won’t have access to your previous user’s documents. You can always exchange files by moving / copying the relevant materials into the Shared folder (inside of the Users folder on the system drive). You can also bypass permissions on files saved on external drives by selecting the drive, and choosing File > Get Info and selecting “Ignore Permissions on this Volume” (changing this setting usually requires Admin authentication).

**This is assuming that the system is functioning correctly and that there isn’t some unknown / unpatched security hole that allows an intruder to bypass the security structure. To my knowledge, the only currently *known* examples of this are a bug that Apple patched in the 10.4.7 update, and the Wireless card exploit documented in another thread. There may be others, and you may get hit by a car crossing the street tomorrow. It’s an imperfect world.