One way you can double check to see if you are the Administrator in XP is if you go to start/run and type "control userpasswords2". In there you'll be able to view the properties. If you are not an administrator you may want to ask the administrator on the pc to give you administrative rights. If the administrator does not allow this then you will have to call upon that individual to change the permissions for you. People like to put user restrictions on their pc's as a security feature or just so people don't mess up their settings. Usually happens in a controlled environment.
This sounds like the case...
--Good Luck