Go to http://www.AffordableCustomSoftware.com (my site). After you look at all the good stuff I'm selling, look under 'Free Stuff' for Inno Setup Visual Builder. It can be used to build an Inno Setup without knowing anything about Inno Setup. One of the wizard pages it can create let's the user choose where to store application data. You can set the default to My Documents or Application Data (which is ProgramData on Win7). If you don't want to leave it up to the user, use the code that Visual Builder generates as a guide to help you learn the Inno Setup coding conventions.

Let me know how it goes. Visual Builder is still in beta, so suggestions are welcome.