Ubuntu and Kubuntu (the variant I use) have emerged as very useable Linux "distributions". They're suitable for computer users who have a little sense of adventure. I turned an old 0.3 Ghz (compare to 3.0 Ghz) eMachine that has a 2.1 GB hard drive into a Kubuntu computer that I use exclusively for business transactions because it's much more secure than Windows. :-)
Installing Ubuntu (semi)permanently is, shall we say, more of an adventure than installing a big program. If you just want to try Ubuntu, there are two ways to do it.
The Ubuntu story is very interesting. An editorial in Free Software Magazine puts it succinctly:
"Then, a very smart man called Mark Shuttleworth made 500 million dollars in the .com boom, learned Russian from scratch, went to space, came back in one piece, funded several charities focusing [sic] on South Africa, and... oh yes, he created Ubuntu Linux."
There are dozens, actually hundreds, of Linux "distributions" -- essentially branches of the core Linux operating system. The nice thing is you can try as many as you like for free. Learn at LinuxLinks.com about which ones (other than Ubuntu/Kubuntu) you might want to explore. One interesting thing is that most Linux distributions come with *tons* of free programs for you to use.
Dan Martin wrote a revealing comparison of the differences between Linux and Windows. By the way, if you're not sure how to pronounce Linux, listen to Linus Torvalds say "Linux" himself.