When you use microsoft power point you may find it difficult to add annotations using default pen as it is difficult to write with mouse. For this you may buy a digital pen that helps you write on screen. But that is bit costly affair. So if you are not professional and just want to start creating video lectures and everything you may not want to spend money on this.
I was in the same boat. And while I was searching for different options online I came across a tool called Zoomit. From name you may feel it is just a zooming tool. But you are wrong. This tool helps you draw regular shapes, arrows,lines and also helps you to write text in variety of size and at any location on screen. You may use it to zoom in and zoom out too. And most importantly it was free!
See the tutorial below about how to use this tool:
Tool can be downloaded at : http://technet.microsoft.com/en-us/sysinternals/bb897434
