- Check SQL Server network configuration for the installed instance: ensure TCP/IP protocol is enabled, activated and listens to the port 1433. Restart SQL service after each change in service/network configuration.
- Try to connect to installed instance locally using osql.exe (to find in %Program Files%\Microsoft SQL Server\100\Tools\Binn):
- Does it work? Try the same from remote machine (i. e. from the client where you experienced connection problems).
- Connection timed out on login? Or another message like "Connection string invalid"? Return to the SQL Express machine and check the firewall settings.
- Ensure firewall rules allow port 1433 as exception.
- Ensure firewall rules allow sqlservr.exe as exception: ensure the path is %Program Files%\Microsoft SQL Server\MSSQL10.
- Check firewall rules on the client: at least the port 1433 should be allowed.
- Retry the connection: does it work now? My client can connect now - and yours?