False "Invalid User Name or PAssword" message

You may have the 'Loss of admin privileges syndrome'. The warning may not mean . the password is invalid but that it is not for what is now an admin. I and many others have had this from Leopard.
Try this, even if you have not enabled root do so now only to disable it after the exercise.
Insert the install DVD in any user. Restart holding down the 'c' key. Wait whilst the machine boots from the DVD. START (only) the install. When you have selected a language, do not proceed to install but pick fro one of the top menus the option in (utilities?) to re set password.
Choose to re set the root password. Think of a good long password with upper/ lower case letters/numbers/ and Shift+top line normal kb characters such as $.
Note said PW. Now choose from the top menus to reboot and reboot from HD. Remove DVD and login to root using PW just set.
In root go into System Preferences, choose accounts pane. Unlock the lock icon using if need be the root id and root PW. Highlight the 'errant' admin. Tick the little square box marked 'allow to administer computer, if it is not already ticked. If you have more than one admin (a good idea to have a reserve, admins can go wrong) repeat the 'tick' process with each.
Log out of root and into a normal admin. Go to directory services and you should find in the top menu and option to disable root. You did just enable it. Disable it.
My recived wisdom is it is better to have a root that has been enabled>> passworded>> disabled, than have a default of a disabled unpassworded root. It is worst to have a continuously enabled.
You now have an admin enabled to admin. If you still have a problem it may well be an invalid password. So come back.
This problem and the one of an (apparent) comnplete los of the admin account are plagueing these forums. If you hav not already done so upgrade to 10.5.2. Make sure you repair permissions before and after aqnd remove haxies first etc.

Did you go to the Accounts in System Preferences after resetting the password for the System Administrator (root)? If not, you need to do that. Again, open System Preferences, go to Accounts, click the lock to make changes. The computer will ask for the name, put in there "root" and then on the password put the newly created password with the install disk. Click the OK button after and that should open the lock icon. Select your username on the left pane and then check the tick box that says "Allow this user to administer this computer." You will receive a message that you need to logout in order for the changes to take effect. Just click OK to that, close the accounts window then logout. As soon as you have rebooted you should now be able to use your own and password.

Thomas,
You have a root account (actually you always have one only it is nowadays disabled by default)and a root password. Log in again as root and go again to System Preferences.
What you have to do now is not to twiddle with the root account but with the admins. Open the little lock. You can do the required authentication using the root ID and password. Root is an admin. Now highlight the admin you wish to sort. When its boxes open., tick the little box 'administer this computer' as an option for the said admin. Do the same for any other admins.
Close the lock. Log out of root. Log into one of your newly tarted admins. Open the application Directory Utility. It is in 'Utilities'. In the 'edit' menu of DU, third from bottom you should find an option to 'disable root' .Disable it.
Now tootle along to System Preferences again, this time you are logged in as an admin. Highlight the admins and check they are said to administer the computer.

Okay, so can you login with your own user name? If you can, we can disable the root later. Simply login to your computer using your own user name and password. Go the the Accounts setting, click the clock and when it asks for a name, put in there "root" and the password you've created. This should open the lock. Again select your name on the left side, check the tick box that says "Allow this user to administer this computer". After that logout and log back in. You should now be the admin.

To disable the root, simple open your applications, utilities and open directory utility. On the menu bar choose Edit and click on the Disable Root User.

Hope this helps

Thomas,
Have you perhaps got two admins logged in at the same time? It could be perhaps that the authentication is rejected as it is not the first admin log in, a bit like the IP conflicts from awakened clients on a network.
See if you can duplicate the particular problem whe only one admin is logged in and then when only one non admin is logged in. In this latter instance try each of the know admins as the authentication. Have you checked that the admin assignement has stuck after the Install Disk re set exercise.
I did see another post where the matter of multiple logged on users was clearly the issue but can't recall it nor it's outcome.

This same problem happened to me, all the sudden one day my mac decided to revert to the original standard root username and password which I obviously did not know. In order to change this:

1) Activate single user mode on reboot as by pressing cmd + s as your computer starts up. Make sure you do it right away, and keep the keys pressed down.

2) Where the cursor pops up, type exactly

+/sbin/mount - uw/+
3) When the next cursor pops up type exactly (note without the letters o and r)

passwd

4) Then you can type any password you want (now changing the root password to be what you want)
+Note: you will not be able to see what you type, but the text is being recognized+

5)The computer asks for you to retype the password, retype it

6) Type reboot

Once the computer restarts you can navigate to system preferences>accounts> and then unlock the account (lock is located on the bottom left corner) using Username: root and the password you just set. Henceforth, you can appoint your account as the administrator. Hope it works!