have a look at download.com and search for CAD software.
There are free CAD programs, but beware, they're pretty crappy and hard to use. Your probably better off getting a second hand CAD program of a later version.
I.e. TurboCAD back to at least V.7 would be better than most free CAD programs.
I asked my Engineering Studies teacher about CAD programs, and he lent me his TurboCAD CDs to install on my computer to use while I was enrolled at my school. (I said I would uninstall them when I finish, but we'll see about that).