If you are running leopard, type this command into terminal and hit return/enter:
sudo rm /private/var/db/.AppleSetupDone
(then type in your password and hit return/enter again)
That will remove the file so that the next time the computer starts it doesn't find the file and goes through the setup again. The account you type this command from must have a password as "sudo" commands require you type your password. Just make sure the account you leave on the computer is some account that your client knows the password to (you'll give them the password) because when they go through the setup they will create a new admin account. They will then need to (they don't have to I suppose) go and delete the other account you used to type the terminal command with. Got it?
PS: If you're running tiger use this command instead: (renames it instead of removing it)
sudo mv /private/var/db/.AppleSetupDone /private/var/db/.setupnotdone