There are many live-on cds such as knoppix and Ubuntu GNU/Linux. Typically with a live-on cd you can download the image and burn it onto a cd. Keep that cd in the cd-rom and reboot your computer, booting off the cd-rom instead of the harddrive. You'll know be using a live-on cd! If you like the operating system, there will typically be an option where you can choose to install it on your harddrive, overwriting whatever is there. If you don't like it, simply turn the computer off, take out the cd and turn the computer back on, it will boot whatever was on your computer harddrive initially.