First off, if you want to share data, then don't proliferate SQL instances: use a single instance that is visible to all machines that need to share the data, and don't attach a database - create it in the SQL server instance and use that. Attaching is a different process entirely, and it creates a "local copy" for your app's use only:
Bad habits : Using AttachDBFileName - SQL Sentry[
^]
If you don't want to share data, then don't use SQL Server: use Access, SQLCE, or SQLite instead as they require no server installation.