Linux is an open-source operating system and is an excellent alternative to Windows.  Open source means that the source code is freely available and also means freely downloadable.  The first step in using Linux is choosing a distribution.  A distribution is a compilation of several Linux applications and the Linux kernel.  Most distributions keep up-to-date with the latest Linux kernel, so it is really a matter of personal choice which distro you use.  A one-stop shop for Linux distributions is www.linuxiso.org.  Here several of the most popular distributions are available in iso format.

My distro of choice is slackware.
Pictured below is slackware 8.1 running KDE.