Problemas con la automatizacion de respaldos en sql express 2005

[iislas]

-Sserver_name [ \instance_name ]

Specifies the instance of SQL Server to which to connect. It sets the sqlcmd scripting variable SQLCMDSERVER.

Specify server_name to connect to the default instance of SQL Server on that server computer. Specify server_name [ \instance_name ] to connect to a named instance of SQL Server on that server computer. If no server computer is specified, sqlcmd connects to the default instance of SQL Server on the local computer. This option is required when you execute sqlcmd from a remote computer on the network.

Si la INSTANCIA ES NOMBRADA (que tu le pusiste un nombre), debe llevar el \\nombreserver\instancianame

Si no la nombraste, entonces debes especificar unicamente le nombre del servidor, se entiende que la instancia es la de default