Useful links for WIX UI.

WIX provides you with sets of built-in UI dialogs. To add a dialog set, add the reference the WixUIExtension:

light Product.wixobj -out Product.msi -ext WixUIExtension 

Built-in dialog sets in WIX Tutorial

But what if I need to customize my installation?

The following articles could be usefult to understand how to customize installation dialogs:

Wix Tutorial - Lesson 8

Extending UI

Windows Installer - User Interface Reference

Will try to consolidate all this information into the one article.