Error message received by Visual Studio 2005:
"Connections to sql server files (*.mdf) require sql server express 2005 to function properly"
I had installed Visual Studio 2005 and SQL Server 2005 Developer Edition when I received it. To solve the porblem I installed other instance of SQL Server - Express Edition. But to get VS to work with it I had to change its instance name in the VS options. Tools -> Options - > Database Tools -> Data Connections. Now I can work with the two sql server instances - Developer Edition and Express Edition.
To change an existing instance of Microsoft SQL Server 2005 to a different edition of SQL Server 2005, you must run SQL Server 2005 Setup from the command prompt and include the SKUUPGRADE=1 parameter.