Is there a way to produce a diagram showing existing tables and their relationships given a connection to a database?
This is for SQL Server 2008 Express Edition.
Yes you can use SQL Server 2008 itself but you need to install SQL Server Management Studio Express (if not installed ) . Just right Click on Database Diagrams and create new diagram. Select the exisiting tables and if you have specified the references in your tables properly. You will be able to see the complete diagram of selected tables. For further reference see Getting started with SQL Server database diagrams