renaming a sql server with installed named instances

We have a hosted server that is named, by the hosting company, as something like WinEnt99. We would like to rename it to something more meaning like OurMonitor1.

We have already installed a few instances of SQL Server 2008 R2.

Is there anything that we need to (re)configure in SQL Server / SSMS ?

I have an application that was reading the database settings from the registry, and connecting the database fine. Now, it won't connect to the database. The only thing that I can think of is that we renamed the server last week sometime.

EDIT 02 Oct 2012:-
The Server is being renamed. The named instances are not.

I've run

sp_helpserver
select @@servername

Which showed the old server name,
and

sp_dropserver 'Old Server Name\Instance_name'
go
sp_addserver 'New Server Name\Instance Name','local'
go

Then confirmed the details in

sp_helpserver
select @@servername

You are only allowed to change the server name or hostname of a server running SQL server. Microsoft does not support renaming the instance without a reinstall. So if you have a default instance on the server that is referenced by the hostname you can rename the "instance" in that manner. However if you are wanting to go from "hostname\instance1" to "hostname\instance2" you will have to reinstall SQL Server.

StackExchange Database Administrators Q#25214, answer score: 4