This is a very common problem after you do a new install of SQL Server on a server. Moment you try to access it over the network, following error pops up in one form or the other.
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 Inrerfaces, error:26 - Error Locating Server/Insatance Specified)(Microsoft SQL Server)
In SQL 2005, there used to be surface area configuration manager that you could use to enable network access to your SQL server. In SQL Server 2008, the interface has been changed a little bit. There is no surface area configuration manager per se. Here is the procedure you will follow in SQL 2008.
If you are trying to connect to named instance, make sure that you are specifying correct instance name and that instance name actually exists on target server and is running.
Make sure that SQL Browser service is running on target server. Otherwise you will get the error that I have mentioned at the start.