On a Mac

- Open up QuickTime Player

- Click the record button

- Click to record the full screen. Drag to record part of the screen.

- End recording by clicking the stop button in the menu bar.

- Save the file

On a PC

- Download Microsoft Expression Encoder 4 ( Link )

- Open it

- Click red record button

- Drag edges of capture window

- Click Record

- Click Stop when you're done

- Save the file