I believe someone already mentioned it, but the standard ubuntu install cd also
works as a live CD. You just pop it in, boot the computer from the CD, then
select "try ubuntu without making c,hanges to your computer." It will be slower
than a full install, and slower than a puppy linux live cd, but will give you a
better idea of what the ubuntu OS is like before you commit to it. Ubuntu is,
in my experience, quite different than puppy linux. However, i do like puppy
quite well for my much older hardware.