Windows 2k Pro Security

I created a user account on this 2k machine locally as a “restricted user” group. I re booted and logged in as the user, the permissions were as expected. However, I was able to go back and change my own group level to “Administrators” machine told me to log off and back on, I did & POOF!! I had Administrator group permissions! Is it something I did wrong on the Server? or is it Windows?? Server is Advanced Server w/SP4,& Active Directory,workstations are 2k Pro. I can do this on all other workstations, so if I can do it, so can my users. Anybody seen this before? Can I fix it? Please advise, thank you in advance.