The reason they cannot work is because Windows only supports dual modes for the target platform, that is, for Windows 64bit only 32bit and 64bit apps are supported, not 16bit - so for instance the 16bit AutoCAD 2000 installation program cannot run. That said, Windows 32bit supports 16bit and 32bit, not 64bit - so the 16bit AutoCAD 2000 installation program *can* run. The problem with some versions is that the installation program is a 16bit program, and 16bit programs cannot work on Windows 7, 8, 8.1 and Vista 64bit. Older versions of AutoCAD are all 32bit applications which means that it is totally capable of running on Windows 7, 8, 8.1 and Vista 64bit. Important Update! Users of AutoCAD R please read these updated AutoCAD R installation instructions (look just above here) instead of the following:
How to manually copy an AutoCAD install from an XP machine to a Windows 7 64bit, Windows 8 64bit, Windows 8.1, Windows 10 or Windows Vista 64 bit and make it work! (Advanced users only)