How do I change the instance name in MS SQL 2005 without installing a new instance?
It's not possible.


  You need to reinstall it. There is no
  instance rename facility for the
  Database Engine.


http://social.msdn.microsoft.com/Forums/en/sqlsetupandupgrade/thread/d5d297b0-2995-46ba-9d3b-d1ec8825f479