Append to the Local Administrators Group via GPO
Previously when you used a GPO to restrict members of the Administrators group using Restricted Groups, the GPO would wipe out all entries and replace it with what you had in your GPO. This made adding an individual person to the local Administrator's group a real pain. Apparently as of Windows XP SP2, the behavior of this GPO has changed and now it appends the entries in Restricted Groups rather than replacing the entries.
There is even a hotfix available to make this work for Windows XP SP1 machines. Check out the knowledge base article here: