you can use autocad on a 64bit system.
if you are a student, you can sign up with autodesk for free software. check with them for more details on this.
autocad is a paid software. you cannot get its full version for free. you can only use the trial version for a limited period. what you're asking for may be considered piracy and tomshardware does not support piracy. it is against forum rules. read the rules here: http://www.tomshardware.com/forum/38307-63-read-first