Recently i deployed SQL Server 2008 along with SP3 on Windows Server 2008 R2. All installation and later configuration were managed through Remote Desktop. Later while trying to connect the instance on LAN from my desktop PC i got following error message
The generated error message is related to network access of instance. Back through remote desktop, first of all i checked for firewall issues but every thing was fine for that. Then after little exploration of SQL Server configuration manager i observed that TCP is disabled. I enabled the TCP in configuration manager as shown in snap
A warning box appeared suggesting the SQL Server service restart for changes to take effect.
I restarted the service from configuration manager and again tried login from desktop PC on LAN. This time login was successful without any error message.
So make sure to enable TCP-IP for accessing the SQL Server 2008 instance on LAN. TCP-IP is by default disabled in SQL Server 2008 installation.