How to: Rename a Computer that Hosts a Stand-Alone Instance of SQL Server 2005
1. For a renamed default instance, run the following procedures:
Get SQL SERVER NAME: select @@servername
sp_dropserver <old_name>
GO
sp_addserver <new_name>, local
GO
Restart the SQL Server instance.
2. For a renamed named instance, run the following procedures:
sp_dropserver <old_name\instancename>
GO
sp_addserver <new_name\instancename>, local
GO
Restart the SQL Server instance.
URL: http://msdn2.microsoft.com/en-us/library/ms143799.aspx