There's a group out there that used to work on a project called "DemoLinux". Basically it's one CD you download and then you can boot off it to run Linux. It runs solely on RAM drives, so installation is no problem. To get out of it, just reboot your computer.
http://www.demolinux.org/
Unfortunately, there was some legal issues with what they were doing, so this hasn't been updated in over a year. Still, I like to keep a CD on hand for showing off Linux to my non-tech-savvy friends.