was having a problem trying to attach/add a db file of SQL Express 2005 to the Visual Web Developer 2005 Express App_Data. The error is as below:

 

 

It seems like VWD 2005 failed to find a valid SQL Express 2005 installed. This doesn't happen to my PC at home. The difference is my PC at home has SQLExpress installed as default instance where my office PC has it as MSSQLServer.

 

Solution:

 

in VWD

Tools -> Options -> Database Tools -> Data Connections (make sure show all settings at the bottom left is checked)

 

SQL Server Instance Name (blank or default): <change it to the instance of your SQL Express> or blank in my case because i do not seems to have an instance name

 

Picture attached as example

 

and it fixed my problem