If you're a Windows user who just wants to see what Linux looks like to decide whether it's for you, the easiest thing to do is run it from a live CD -- a version of Linux that boots directly from a CD without ever changing any bits on your hard drive.
Where do you get live CDs? You can download a live CD image from our site or from FrozenTech, and save the file on your desktop. You must then burn it to a CD with your favorite CD writer application, pop it in your CD drive, and reboot. When the system comes up, press the key that displays your system's BIOS and make sure that the CD drive appears before the hard drive on the screen that controls the boot order of the devices on your computer. Save your changes if necessary and continue to boot. When the computer starts to come up, you'll see it start running Linux.
A live CD is good as a demonstration (and some special-purpose live CDs are good for system management tasks) but not so good at running a regular system. Because they run from a CD drive, they're slower than hard-drive-based systems, and it's much harder to customize them and add new programs. Therefore, a better option is to install Linux on your hard drive in a dual-boot configuration. That means that all your Windows programs and files stay right where they are, but you add a new Linux area to your drive. Every time your system boots, it will ask you which operating system you want to work in. You can choose which one it defaults to if you don't make an explicit choice.
Which version of Linux should you choose to install on your hard drive? We suggest you start with one that's free of cost. You can try distributions from multiple sources if your first choice doesn't suit you -- all it will cost you is the time.
Reams of paper and hundred of Web pages have been devoted to the question of which distribution is best for Linux users, and for beginners in particular. Rather than delve into the pros and cons of each distribution, we're actually going to answer the question:
Get the latest version of Kubuntu.
Kubuntu is based on Ubuntu, one of the most popular and actively developed distributions today. However, while Ubuntu uses the GNOME desktop environment, Kubuntu uses KDE, which will be much more familiar to Windows users. Kubuntu is easy to understand and use, and because it has a huge and active user community, you can get answers to technical support questions quickly.
(OK, we can't help but offer a little wiggle-room. If you don't like Kubuntu, other free desktop distributions that should be easy for Windows users to take include SimplyMEPIS, Freespire, and Mandriva One.)
Remember, even though you're installing Linux, you're not getting rid of Windows. A good resource for walking you through installing Linux in a dual-boot configuration is on the Ubuntu wiki.
If you don't mind spending a little money on reading material to give yourself a head start on Ubuntu, one good reference is The Official Ubuntu Book. There are many other worthwhile works on Ubuntu specifically and Linux in general.