Microsoft SQL Server Login Connection failed

After upgrade from 2005 to 2007, some workstations get this error:

Microsoft SQL Server Login

Connection failed

SQLState; u201808001'

SQL Server Error: -1

[Microsoft] [SQL NATIVE CLIENT] SQL Network Interfaces: error locating server/instance specified [xFFFFFFFF]

Connection failed;

SQLState: 'HYT00'

SQL Server Error: 0

[Microsoft] [SQL NATIVE CLIENT]Login timeout Expired

Connection failed;

SQLState: '08001'

SQL Server Error: -1

[Microsoft] [SQL NATIVE CLIENT] an Error has occurred while establishing a connection the sql server. when connecting to sql server 2005, this failure may be caused by the fact that under the default settings sql server does not allow remote connections