I was doing some reading tonight and I found this absolutely cool article on creating custom effects for Windows Movie Maker in XML and C++. Check it out here:
Creating Custom Effects and Transitions in Windows Movie Maker
Microsoft Windows Movie Maker enables programmers to add their own custom special effects and transitions to the Windows Movie Maker interface. These effects can be created with an XML file that modifies the supplied effects or transitions, or can be built from scratch using C++ and the Windows Media Transforms supplied by the Microsoft DirectX Media SDK.