OS X 10.5 Leopard - User Lost Administrator privileges

Some days ago I had some updates available for my Leopard but couldn't install them as I was not
allowed to administer the computer for some unknown reason.
Apple says that it's an issue with Leopard 10.5 and tell you how to fix it with installation DVD and 9 steps, see the tutorial here:
http://docs.info.apple.com/article.html?artnum=306876

I found another solution only based in command line, offered by macXORHacker:
Here is the fix.....
-> Shut down the computer if it is on.
-> Press the power button to start the computer.
-> Immediately press and hold the Command (Apple) key and the "s" key for single-user mode.
-> Type "mount -uw /" and press return.
-> Type "passwd" and press return.
-> Enter new password (this will be for the root user account) and press return.
-> Type "reboot" and press return.
-> Enter Account settings and when prompted for administrator account and password, use the user name root and the password you just setup
-> Check box for you standard account to administrate box
-> If all goes well you are admin again.
I will add to this, please DISABLE the root account after you do this, or at very least, use a good, strong password."

And that did the trick for me.