"Draw data flow diagrams"

Whether Data Flow Diagrams (DFDs) are the most appropriate solution depends on the nature of your application.

For control-oriented applications State Transition Diagrams (STD), or State-Event Matrices can be more natural.

For program structure, you can use Structure Charts!

As Michael said, it can all be done in Visio - at a fraction of the cost of a dedicated CASE tool.

You can even do UML in Visio - and it does give you more than just the pictures.