Q:

Error after removing sql server 2008 R2 Express edition and instaling Entreprise edition: Unable to connect to MyHost\MSSQLSERVER

So I'm having this issue after installing MS SQL Server 2008 R2 Enterprise edition, I'm getting this message error:
Unable to connect to WIN7 \ MSSQLSERVER

A network-related error or instance-specific 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 : 25 - Connection string is not valid ) (Microsoft SQL Server, Error: 87)

NB: Before installing this version i did remove an existing MSSQL 2008 R2 EXPRESS, but after launching "SQL Server Management Studio" i strangely got for the first time the following instance of the express edition version: WIN7 \ SQLEXPRESS
My configs are: Windows 7 Enterprise Edition SQL SERVER 2008 R2 Enterprise
the SQL connection session: - DB Engine - WIN7 \ MSSQLSERVER - Windows Authentication
Please HELP !!

A:

For a default instance (MSSQLSERVER), you only specify the computer name when connecting; the instance name (which is actually a ficticious placeholder) is implied.
Also, if you're connecting from another machine now, you may need to configure the Windows Firewall to allow connections through. The error message you get will be the same as the one you're seeing now. See my video here for more on that.

