Victor Riley's new blog, directly named "Thoughts on Human Factors and User Centered Design", recently featured a how-to on Product Prototyping with PowerPoint (try repeating that five times quickly).
The article discusses the utility and relative robustness of PowerPoint for basic product interface prototyping (a clothes dryer control panel in the example). Of particular interest for researchers is the ability to record prototype use in real-time, utilizing PowerPoint's built-in Visual Basic for Applications capabilities. An example file is referenced to download.
Incidentally, I tried it out in PowerPoint 2007 and it worked fine.