SQL 2008 R2 error: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified

after some year work with SQL server 2005,i decide change to  SQL server 2008 R2.
When i completed install SQL 2008 R2,i can't connected to SQL server due to this following 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) (.Net SqlClient Data Provider)

And when i click to technical details,i take some thing like this
=================================== Cannot connect to HUY-PC\SQLEXPRESS. =================================== 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 c ...