Came across a neat trick in Powerpoint 2010.
If you hit F5 to run your presentation, you can then right-click on the slide:
Pointer Options / Pen
and then draw some annotations or arrows or whatever to help explain. You can also use this to highlight various pieces of text on the fly. Naturally, you can change the pen colour.
When you finish the deck, you will be asked if you want to keep the annotations.
However, if you start with a blank slide, you can use this to physically draw Powerpoint slides that are only pictures, arrows etc. in freehand.
Build up the blank slides, say "Yes" to keep annotations and voila - a freehand presentation.
There are keyboard shortcuts e.g. Ctrl P for the Pen, Ctrl E to Erase, Ctrl M to show / hide the markup etc.
There's a full list here.