I'll recommend Ubuntu too, based on your (short) post. But yes, there is a huge number of live-cds out there (and dvd versions too I guess); it's then a completely different matter which ones of them suit you
Ubuntu is anyway a good start I think, it's got most of the things you'd expect to see in a freshly installed OS while still looking "friendly" or "simple". Knoppix is then a nice "swiss army knife" handy to have around, with lots of programs to use when your machine doesn't boot and you'll want to do something about it..not that Ubuntu couldn't do those things as well, but Knoppix was the first live-cd I used so it became some sort of a tradition then
Some projects also offer a live-cd though they're not distributions as such; for example Gnome site has a link where you can obtain a live-cd with the new Gnome 2.22 on it, to be able to try the new desktop environment out. It's basically some distribution packed with new Gnome, but the main focus is in the desktop, not the distribution - whereas Ubuntu livecd main focus is in Ubuntu, not Gnome
well, try to sort that out then..my bad, if it was confusing.
Get the Ubuntu livecd for a start, and if it feels absolutely terrible, try something else - and do get a 700MB cd-rw for the job, so you don't have to waste cd-r discs if you later choose to try something else out. The price difference between a -r and a -rw disc is so small nowadays that you don't actually "lose" anything buying one if you don't already have it.