C# .NET - unable to attach database error

 A database with the same name exists, or specified file cannot be opened, or it is located on UNC share. 

  I'm creating a C# windows application, with 2008 sql server management studio in one machine that'll hold the database and the clients can access the database via lan.

  how to do this, Could Someone help me?
I'm new to this kind of things..

You connection string should look like...

Data Source=.\SQLEXPRESS;Database=<your database name>;Integrated Security=SSPI

The error you're getting is because VS generated a User Instance connection string.

A User Instance is a separate instance of SQL Express that is started at runtime which tries to attach the database specified for use by the application. Your database is already attached to the parent instance of SQL Express, and databases can only be attached to a single instance at a time.

By changing the connection string, you'll be using the parent instance, where the database is already attached, rather than the User Instance.