Originally Posted by shaggysaurusrex
First, a basic fact: 64-bit Vista is unable to run 16-bit software. Completely unable. Will not work, period. It's just how Microsoft designed the system.
32-bit Vista can still handle old 16-bit executables, however.
Now then... your error message seems to be indicating that the Autocad installer is a 16-bit installer. Since Vista x64 cannot handle 16-bit programs... it cannot run the installer, even if the actual application turns out to be 32-bit.
The only chance would be if you could actually copy the program directory from another machine - on older software that has been known to work, though I would cross my fingers for that one.
Otherwise, Autodesk would have to port the install script to a 32bit or 64bit installer. Not gonna happen. Keeping a few legacy customers happy isn't worth that kind of resource investment - not when they can simply tell you to upgrade to the latest version and then get some more money from you.
Sorry to be the bearer of bad news... but your chances are slim to none.