AutoCAD 64-bit cannot be installed on a 32-bit Windows Operating System.
The System requirements for Windows Vista or 3D modeling (32 and 64-bit AutoCAD)
* IntelŽ 3.0 GHz or greater * 2 GB RAM or greater * 2 GB free hard disk available not including installation * 1280 x 1024 32-bit color video display adapter (True Color) 128 MB or greater, OpenGLŽ, or Direct3DŽ capable workstation class graphics card. * For Windows Vista, a Direct3D capable workstation class graphics card with 128 MB or greater is required