I came across a website that provides PowerPoint templates for prototyping your Windows Store and / or Windows Phone apps. These templates are NOT free, but they may be helpful in getting your prototypes done quickly. (Note: I haven’t tried any of the templates and therefore cannot offer my personal experience in terms of how useful they are.)
If you have an idea for a Windows Store or Windows Phone app, and look for some guidance on how to build an app off of your idea, take a look at the Generation App program below. Sign up to take advantage of the free offering from Microsoft.