Star Power: Security was tight when Elizabeth Taylor arrived at a Hollywood recording studio this week to work on an episode of the animated series "Captain Planet and the Planeteers." The storyline involves AIDS and features Taylor as the voice of a mother of a teen-age son who has tested HIV positive. Neil Patrick Harris of "Doogie Howser, M.D." will be the voice of the boy, and Danica McKellar of "The Wonder Years" portrays his girlfriend. The episode will air in the fall on TBS and in syndication.