Under Windows 7 and Vista, with UAC enabled, part of the installation sequence is run with elevated rights, part is not, and your custom actions are running not with elevated rights unless they are ‘deferred’ but then you don’t have access to the custom table anymore. That turned out less straightforward than I thought.

WIX supports the idea of custom actions that you can write in C#, So I set out to write such a search & replace custom action that was to be configured by a custom table. Now this is usually enough, but if you want to change plain text files files then you are basically on your own. Trouble is that the standard tasks only know how to modify XML files. Unfortunately good examples are few and far between, and I hope to save some poor sod a lot of time with this article It includes all kinds of tasks to modify files after installation, and thus you can for instance change the settings in configuration files based upon user input. This is a powerful albeit a bit complex technology that allows you to build MSI setups using XML files.

Recently I had my first encounter with actually writing setups myself using WIX. WIX configurable search & replace custom action for text files

Wix formatted file identifier