Without an operating system – Mac OS X, say, or Windows XP – a computer is just a dumb box of chips, and even the most exciting application will have no environment in which to run. And, it's easy to forget that you have a choice of which OS you run. In fact, as a Mac user, you're uniquely positioned because an Intel Mac is the only computer in the world that can legally run all three major operating systems, Mac OS, Windows and Linux. Indeed, you can, if you like, load all three on the same computer and pick which one you want to load when you switch your Mac on.