I am looking for the best software product that let's you design software. A tool that let's you express the architecture of a software module in a computer screen, visually.
Sounds like you need to discover the "joy" that is UML (unified modeling language).
http://en.wikipedia.org/wiki/Unified_Modeling_Language
Many tools out there support this ( http://en.wikipedia.org/wiki/List_of_UML_tools ) with the most well-known probably being IBM's Rational Rose.
For UML Rational Software Modeler is probably the best known one. MS Visio is another, and VS 2010 includes it as well.
It should be noted however that UML has gone out of Vogue, probably for good reason.
My personal software development design is VB.NET.
I would suggest downloading one of Microsoft's Express programing languages. You can download it at:
http://www.microsoft.com/express/Windows/
You then have access to C#, VB.NET and various other languages including web design capabilities.
Visual Studio 2010 allows you to do UML.
try Enterprise Architect , provides complete traceability from requirements, analysis and design models, through to implementation and deployment ... , but it not free ,
Architecture Studio is a pretty good tool. It's designed to work with the Architecture Centric Design Methodology and emphasizes the creation of well-documented designs (e.g. consistent design perspective, recording rationale for design decisions) which focus on satisfying architectural drivers. You can specify your own notations and the tool provides some traceability among quality attributes and constraints with architectural elements.
ACME out of Carnegie Mellon is a plug-in for Eclipse which provides a graphical environment on top of an architecture description language. One of the big advantages with this tool is that you can perform model checking on your designs to ensure your architecture adheres to the styles or patterns you chose for your design. So if you say you are using a client-server architecture, the tool will let you know if you are violating the style. ACME also makes it easy to decompose components or modules - it's easy to drill down architectural elements to provide additional details.