XP SP2 – Administrator lost authority!

XP SP2 Desktop PC. When I try to apply a startup change of any kind using MSCONFIG, it pops up telling me I need to switch to a user with Administrator authority to do the change. I have only 1 user defined and it does have Administrator auth. I then added a new user with Administrator auth and logged on as the new user, but same problem. This is a new problem – used to work just fine. Any fix? Charlie