Windows Installer issue

I have Vista x64 Ultimate

Issue: The highest level problem I'm trying to resolve:
I'm trying to install AutoCAD and the install dialog box has the software grayed out on the 2nd step and the Next button is grayed out. That looks kind of like the software is already installed, but it isn't.

• This is after uninstalling the demo version with the OS uninstall tools in the Control Panel...
• The demo version has something going on that prevents a 2nd installation (so that someone won't continuiously uninstall and reinstall and never buy it).
• I manually scrubbed out all the directories as recommended by Autodesk, and I still can't install.

1. I tried a "safe mode" install and Vista reports a problem with the installer.... Is that normal? Should I attempt to reinstall the Windows installer? I'm having trouble locating the best location and version to download that from the Microsoft website.
2. Is there a recommended registry cleaner? I suspect that Autodesk has embedded some setting deep in the registry and that might be a way to scrub them out. (Their manual uninstall routine did not mention the registry).

Any other input is appreciated.