This is good advice.
http://ubuntu.com has step by step to burn a "live CD". Once you burn it, you can put it in the tray of a computer and start it up, and Ubuntu will launch from the CD without touching the hard drive. It lets you try before you buy, although it's very slow. Can also run it from a 2gb memory stick. Also very good way to make a 3 or 5 year old computer useful again.
H.