Since you don't really have to buy Linux, the heading can be misleading. But we've all heard of the statement that somebody might want to "try" something, before they "buy" it. This is where Linux excels over other operating systems like Windows. When have you ever been able to try out Windows, before you decide whether you want to upgrade or keep it?
Most Linux distributions today have a "Live CD", which is a complete running version of the distribution that can run from a CD. This means, you can take a PC currently running Windows, stick in a Live CD of any Linux distribution (Fedora, Red Hat, SuSE, Ubuntu), boot to the CD and see how it runs on your system. This also gives you the opportunity to open up some of the applications bundled with the distribution, and should even give you access to your Windows partitions (since Linux can open partitions of many different types). It's a great way to test drive everything, if you are considering installing Linux on your PC.
One drawback to running from a CD is the horribly slow read speeds. This will definitely not help you get an accurate speed benchmark for your PC. However, this can be worked around by creating a Live USB stick and booting from that instead. This will speed it up significantly, however depending on the architecture of your system it still might not be a truly accurate measure of overall speed. However, the benefit of using a Live USB stick is that you can change settings and have them become permanent, so that you can boot up later to the same Live USB stick and resume where you left off from before.
Using a Live CD or Live USB stick is so powerful that they can be used to fix broken operating systems like Windows or if something happens to your Linux operating system as well. Distributions like Fedora / Red Hat, have a "Rescue CD" available that does just this, and includes extra repair tools.
There are tools available to create a Live USB stick from Windows, Mac OS X, and of course Linux. The nice thing again, is that you can test drive the Linux distribution, and begin an installation right from within the Live CD if you wish. Or, if you are unhappy for any reason, you can simply stop using the CD or USB stick and go back to where you were before. A full money-back guarantee, except that you still didn't pay for anything.