For users who specifically need features, the safest route is to install it legally on an external SSD using Windows To Go (a full portable Windows installation on a certified drive). You then boot from that drive and run standard AutoCAD.
The search for is a journey down a rabbit hole. While the idea of a 150MB, click-and-run CAD program is seductive, the reality is plagued with stability issues, security threats, and legal troubles. Autocad Portable 2012 64
The golden rule of portable software remains: If it sounds too good to be true (full AutoCAD 2012 64-bit for free, no install, 200MB), it is almost certainly malware disguised as a CAD tool. Stick to legitimate lightweight alternatives or invest in a modern laptop that can handle a proper installation. For users who specifically need features, the safest