I don't think trial version of Autocad is available for download. Kindly let us know what you want to do with auto cad. If you just want to open a dwg or dxf files, you can download the viewers from the company website *www.autodesk.com
For basic editing oand creating basic drawings, you can download trial versions of Autocad Alternatives like TurboCAD, CADian, from below sites
*www.cadianusa.com/
*www.turbocad.com/