That isn't normal. If you right-click on My Computer and choose Manage from the right-click menu, Computer Management opens. In Computer Management, go down to System Tools > Local Users and Groups > Groups, and what usernames are in the Administrators group, exactly?
If the account you changed is the one named "Administrator" itself, that is the system's own Admin account, normally hidden. Sometimes mishaps occur and people end up riding around in the system's own Administrator account without realizing it. I don't know if you can de-Admin that one the way you can a normal, user-created Admin account.
As a test, create another account, switch that one to non-Admin, and try using it to do stuff like changing the system time, running Defrag, or creating a file in the C:\Windows\System32 directory. It shouldn't work. I'm assuming your HDD is in NTFS format (NTFS = good).