Depends what you mean. There are hundreds of desktop distros, but linux is much more popular than you would imagine. Android, the Wii, almost all servers, most checkout monitors, and many public displays (airports, fast food menus, etc) all run linux. So, there are a few hundred distros, but I'd estimate there are over 10000 "versions" in use across the world.
AutoCAD is not developed for Linux. Some versions will run in Wine, but most recent versions will not.
Not all version of Redhat Linux is free but you can download many desktop versions free.
They don't.
The question is too vague to answer clearly. There are too many versions of Linux and Unix and third party vendors.
Many distributions have older disk images in their archives. Try there.
Distributions
"Distros", or distributions.
Generally yes. Most Linux versions will work on a standard home PC, and many will work on one too old and slow to run Windows.
No.
Some versions of CorelDRAW can be run on Linux using Wine, while others can't.
For older versions it is "/etc/lilo.conf" and for newer versions "/boot/grub/grub.cfg".
All versions of the Linux kernel are "full featured." The latest stable version of the Linux kernel as of May 17, 2011 is 2.6.38.6.