Richard Foulkes: ;if you want to record your screen use you'll need to sign up but its free to download mp4s, or you can use ; which will take screenshots of stuff in the browser if you want pics of websites etc.
Ian Dixon: The simplest free screen capture on Windows at least is to use Shift-PrtScr to copy to the clipboard then use the GIMP to cut out the parts of the image to use. Windows Movie Maker can be used to put things together.
If you want to actually record video then Camstudio works fairly well. Again use Windows Movie Maker to stitch things together.
Jing is good for screenshots but uses Flash for video. That is problematic for recording because the resulting video may fail to load on the iPhone or iPad. (Apple dislike Flash video and Linux is less than keen)
I've been known to use OpenOffice/LibreOffice Impress to make up a presentation then record that using Camstudio then stitch in titles and credits using Windows Movie Maker