You can download Linux for free from many, many places. Ubuntu is the only one I have heard of that sends free CDs. Another option is the CDs and DVDs that come with magazines. You can buy CDs from many sources for just a few $.
20GB is plenty to install Linux. If you want dual-boot with Windows, it will get tight.
"Linux 7.1" or "6.1" is ambiguous--are you talking about RedHat, SUSE, etc.?---ie what is the name of the Linux you have?
is a good place to get oriented. The "getting started" link below might help also.