AutoCAD Architecture 2008 is not supported for 64-bit OS like Vista 64-bit and the installer is even locked for only 32-bit. But there is a nonsupported workaround. Here are instructions how to edit the ACA.msi file using Orca:
I really from experience can see the need to have applications like ACA 2008 and AMEP 2008 supporting 64-bit Vista because much more memory can be accessed and when working with large models or many files open at the same time this is really useful. 3 GB is recommended according to the system requirements but having more than that would really be useful for many.
Update 4-MAY-2007: If you contact Autodesk Support you can now obtain the files and installation instructions needed to solve this.