Are you looking for 2D CAD software or a 3D solid modeler? I'm not sure of your experience level in CAD so I would recommend;
for 3D solid modeling something like CoCreate (free)

http://www.cocreate.com/modeling_pe_...FQTAXgodGy4m8Q

For 2D there are lots of shareware programs at these links
http://www.caddepot.com/

http://www.tucows.com/Windows/DesignTools/CAD/

http://www.caddprimer.com/Free_CAD_s...e_freeware.htm