Microsoft SQL Server, Error: 11001

Hi All,
in my PC i Install SQL Server 2008 R2 Express Edition and in My Main Server is Installed SQL Server 2005 & SQL Server 2008 Express Edition
when i Connect to SQL Server 2005 through my Computer it will connect without problem and when i Connect to SQL Server 2008 through my Computer it will show my ERROR BELOW
TITLE: Connect to Server ------------------------------ Cannot connect to MAINSERVER\SQLEXPRESS. ------------------------------ ADDITIONAL INFORMATION: 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: TCP Provider, error: 0 - No such host is known.) (Microsoft SQL Server, Error: 11001) For help, click: http://go.microsoft.com/fwlink?ProdName=Microsoft+SQL+Ser ...