SQL string connection issue

SQL string connection issue

I have to establish a connection using connection string to DB. I am using
SQL Server 2008 R2, for which i need connection string. It is not
SQLEXPRESS.
Current String::
< add name="ApplicationServices" connectionString="server=USER-PC\SQL
Server;database=TLE;Connection Timeout=40"
providerName="System.Data.SqlClient"/>
But it is not working. I am getting error ""A network-related or
instance-specific error occurred while establishing a connection to SQL
Server. The server was not found or was not accessible. Verify that the
instance name is correct and that SQL Server is configured to allow remote
connections. (provider: SQL Network Interfaces, error: 26 - Error Locating
Server/Instance Specified)""