I had SQL Server 2005 Express Edition installed along with Visual Studio 2005. Today, I installed SQL Server 2005 Developer Edition, but installed it as the default instance instead of as a named instance. Probably as a result of that installation, I’ve lost the Express Edition. Is it OK to install SQL Server Express on the same machine as the SQL Server Developer Edition?

Yes, I have them side by side on several machines. Are you sure you lost SQL Server Express? Look in the Administrative tools, Services menu. Typically, SQL Server Express is installed as a named instance called SQLEXPRESS, but you might not see the SQLEXPRESS named instance when SQL Server Management Studio first opens. When you see the instance, you simply need to establish a connection to Database Engine\<server name>SQLExpress.

If you did overwrite your SQL Server Express instance during your installation of the Developer edition, then yes, you can add SQL Server Express as a named instance. Just download the Service Pack 1 (SP1) version at http://www.microsoft.com/sql
—Richard Waymire
Program Manager Architect
Visual Studio Team System