Como conectar RED SQL EXPRESS con c#

Hola, no consigo conectar una aplicacion hecha c# con SQLEXPRESS en un red
local me dice que no encuentra el servidor error 26. he activado el TPC/IP
pero nada
me sale

TITLE: Connect to Server


Cannot connect to PORTATIL_AGU\SQLEXPRESS.


ADDITIONAL INFORMATION:

An error has occurred while establishing a connection to the server. When
connecting to SQL Server 2005, this failure may be caused by the fact that
under the default settings SQL Server does not allow remote connections.
(provider: SQL Network Interfaces, error: 26 - Error Locating
Server/Instance Specified) (Microsoft SQL Server, Error: -1)

No se configurar en red el SQL EXPRESS

Gracias
Agustin