It's not hard to find a free copy of Linux to download. Debian, Mandrake and CNET's download site all provide access to a multiple array of free Linux operating systems.
However before you download there are certain advantages to buying a commercial copy of linux:
1.Unless you have a fast connection downloading Linux will be a pain, a Linux distribution typically is over 100mb download, so make sure you have the time.
2.Linux can have a steep learning curve. This means that you will most likely spend a lot more time just trying to get Linux to work. Then again, not having to see the Windows blue screen of death should be enough to motivate most people.
3.Usually if you buy a version of Linux it comes with a users manual in print, which is very helpful if you don't have a printer.
Windows XP Home on the other hand can cost up from $99 to $189 on Amazon.com depending on if it's an upgrade or not. Even when you buy a copy of windows, with the recent version (Windows XP) you can only put it on one computer.
Linux gives you more control of the operating system and can be customized to a greater extent because it is open source. This is further explored in point #2.