I have been using a new product by Microsoft called Office Community Clips, the screencasting software for Windows (link) can capture all your desktop activity including the keystrokes and audio. You can quickly and easily create videos or screencasts so student can access and review the lesson at a later time,. This is a great way to differentiate your instruction.
When using with PowerPoint you can synchronize your slides and voice when preparing online lessons. If you are using a tablet PC you can record math lessons and post online, allowing students the opportunity to review a lessons. It's quick and easy to learn!
When you install Community Clips, it adds a small record button to the Quick Access Toolbars of Word, Excel and PowerPoint that allows one-click recording of the application.
Office Community Clips Download Link Office Screencast VideosUpdate: Since Community Clips can record audio, you may also use it for recording videos of your PowerPoint slides with the narrations.
Also see: Record PowerPoint Presentations as Video Files