What problems are you having with the flowchart? Are you trying to model the functional flow of the application or the logic flow through the components of the system?
If you're just trying to show how the installer works from a functional perspective, then a flowchart is fine and has the advantage of being understandable by non-technical people.
But if you're doing a technical design, then probably you should use UML class and sequence diagrams. The class diagram shows the static relationships between classes, while sequence diagrams show how the classes interact to implement the functions of the application. You'll generally only need one class diagram but a whole bunch of sequence diagrams, because you need to work out the logic flow for each of the use cases that you've identified.
UML defines other diagram types that are all useful under limited circumstances, but class diagrams and sequence diagrams will get you 90% of the way home at least.
There's a diagram called a collaboration diagram that's related to a sequence diagram in that both show interactions between components. A collaboration diagram is what you create when you draw a bunch of boxes on a napkin and with arrows between them to show how components talk to each other. You may find that starting out with collaboration diagrams is easier.
This is a useful tool for making sequence diagrams:
http://www.websequencediagrams.com/