Linux is a vary broad OS with many flavors(distros). There are some LIVE CD's which are plug and play and will boot from your cd drive without even touching your HD(Gentoo Linux is I believe).
Linux is NOT for everyone. Some poeple don't know a harddrive from a keyboard. Once you know some basics(takes a couple hrs to get the feel of it-Command Line, different layout, customization, updates...).
Knoppix is very small and works great for little comps. To see the full power of Linux though I suggest Fedora Core. I use it everyday and it has never failed me(no crashes - the one that I accidentally f'ed up my graphics card, my fault on that one for experimenting

).
Ubuntu is pretty good from what little I have used it.
--Basically, Linux isn't for everyone and will probably never be. It just isn't that user friendly and everyone doesn't want to put forth the effort. Definitely worth a try though for the above avg. user.