The next version to be released of Ubuntu v11.04 (also called Natty Narwhal) will be available on 28 April.
Ubuntu has been one of the easiest versions of Linux to install for some time now and is usually well suited to older computers that have run out of steam with Windows. It is completely free unlike Windows/MacOS and comes with all the software most people will ever need.
If there is enough interest the group might like to have a Linux installation session if members wish to bring old computers along (first make backups of anything of any importance). Failing that it might be possible to use a laptop and the projector to do a live installation as a demonstration.