here's the free trial version's download link:http://usa.autodesk.com/autocad/trial/
if you're a student, you can register with autodesk and use the free version by following autodesk's guidelines.
from autodesk site:
Students Can Join the Autodesk Education Community
Use the tools that are helping shape the future of design
Get full versions of software
No 30-day trial limitation
Share your designs and view portfolios