1 – enable root from install cd
2 – log in as root
3 – Type this command in terminal “cp /System/Library/DirectoryServices/DefaultLocalDB/Default/groups/admin.plist /private/var/db/dslocal/nodes/Default/groups/”
4 – Restart computer,
5 – Log in as root, open accounts and apply admin prefs normaly.
What is happening is that the admin group is either becoming corrupt or disappearing all together. This restores the original admin group account settings.
The command means – “cp” copy
Location of fresh admin.plist – /System/Library/DirectoryServices/DefaultLocalDB/Default/groups/admin.plist
Location to copy the fresh plist to – /private/var/db/dslocal/nodes/Default/groups/
The solution I suggested is to be used only if the http://support.apple.com/kb/TS1278 article doesn’t help you. Because some people have the problem where you check off the “allow user to administer computer” checkbox and then when you attempt to apply the settings the check box unchecks itself.