The GNU/Linux operating system -- commonly referred to as "Linux" -- is based upon the principles of
free software and as the child of Unix it inherits incredible stability, scalability and power. Interestingly, most GNU/Linux users don't even need to use
antivirus software, because the operating system has been designed from the beginning to be stable and secure -- contrast this with Windows, which is constantly afflicted with truly horrible virii, backdoor trojans, worms, spyware, adware...
ad nauseum.
Best of all, many GNU/Linux companies and organizations provide links to download 100% free versions of the operating system -- simply download the .isos (CD ROM
image files), burn them to CD, and then install it on your computer.
For more information, I highly suggest checking out the following links:
Mandriva Linux
The Fedora Project
Ubuntu Linux
SUSE Linux (from Novell)