When should I use WPF instead of Adobe Flash?

For creating rich web content, Flash is currently the most popular option because of its ubiquity. You can put Flashbased content on a website with confidence that the overwhelming majority of visitors already have the necessary player installed. (And if they don't, it's a very quick download.)

WPF applications can also run within a web browser. WPF has the advantage of better development tools and programming model, a richer feature set, robust control reuse, broad programming language support, and full access to the underlying platform (when security permits). But viewing such content requires Windows and the .NET Framework 3.0 (installed by default on Windows Vista or later).