Completely clean out traces of AutoCAD installations

Sometime when installations or uninstallations fails it is good to completely remove every trace that the application left.
See also TS45252 (This will remove all Autodesk products)
For only cleaning out for example AutoCAD 2008 on Windows Vista or Windows 7 look for these folders:
%PROGRAMFILES%\AutoCAD 2008
%PROGRAMDATA%\Autodesk\ACD\2008
%PROGRAMDATA%\Autodesk\AutoCAD 2008
%APPDATA%\Autodesk\ACD\2008
%APPDATA%\Autodesk\AutoCAD 2008
%USERPROFILE%\AppData\Local\Autodesk\AutoCAD 2008

For AutoCAD 2011:
%PROGRAMFILES%\Autodesk\AutoCAD 2011
%PROGRAMDATA%\Autodesk\ACD\2011
%PROGRAMDATA%\Autodesk\AutoCAD 2011
%APPDATA%\Autodesk\ACD\2011
%APPDATA%\Autodesk\AutoCAD 2011
%USERPROFILE%\AppData\Local\Autodesk\AutoCAD 2011

and these registry locations: (Be careful though when using Regedit.exe)
HKEY_CURRENT_USER\Software\Autodesk\AutoCAD\R17.1\ACAD-6001:409
HKEY_LOCAL_MACHINE\SOFTWARE\Autodesk\AutoCAD\R17.1\ACAD-6001:409
Notice that depending on the locale/language the LocaleID 409 can be something else. See AutoCAD registry details.
If you have AutoCAD 2013 it is R19.0 instead of R17.1
If you have AutoCAD 2012 it is R18.2 instead of R17.1
If you have AutoCAD 2011 it is R18.1 instead of R17.1
If you have AutoCAD 2010 it is R18.0 instead of R17.1
If you have AutoCAD 2009 it is R17.2 instead of R17.1
If you have AutoCAD 2007 it is R17.0 instead of R17.1
If you have AutoCAD 2006 it is R16.2 instead of R17.1
If you have AutoCAD 2005 it is R16.1 instead of R17.1
If you have AutoCAD 2004 it is R16.0 instead of R17.1
AutoCAD Architecture 2008 (ACA 2008)
Autodesk Architectural Desktop 2007 (ADT 2007)
Autodesk Architectural Desktop 2006 (ADT 2006)

Notice that if you have Windows Vista the paths are somewhat different. The same is if you have another OS language installed.
C:\Documents and Settings folder doesn’t exist in Vista. It is replaced by C:\Users and C:\ProgramData. C:\Documents and Settings\<user>\Local Settings\Application Data\ in XP is C:\Users\<user>\AppData\Local\ in Vista. C:\Documents and Settings\<user>\Application Data\ in XP is C:\Users\<user>\AppData\Roaming\ in Vista.

Here are some tips.
%ALLUSERSPROFILE%in XP: C:\Documents and Settings\All Users
in Vista: C:\ProgramData
%PROGRAMDATA%\Autodeskin Vista: C:\ProgramData\Autodesk
%PROGRAMFILES%\Common Files\Autodesk Shared
in XP and Vista: C:\Program Files\Common Files\Autodesk Shared

And then there are folders and files saved per user:
%APPDATA%\Autodeskin XP: C:\Documents and Settings\%username%\Application Data\Autodesk
in Vista: C:\Users\%username%\AppData\Roaming\Autodesk
%USERPROFILE%\Local Settings\Application Data\Autodeskin XP: C:\Documents and Settings\%username%\Local Settings\Application Data\Autodesk
%USERPROFILE%\AppData\Local\Autodeskin Vista: C:\Users\%username%\AppData\Local\Autodesk

The folders can also be found reading from the registry.
HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\explorer\Shell Folders
HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\explorer\User Shell Folders
For example these:
HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\explorer\Shell Folders
Common AppData
in XP: C:\Documents and Settings\All Users\Application Data
in Vista: C:\ProgramData
HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\explorer\Shell Folders
Common Programs
in XP: C:\Documents and Settings\All Users\Start Menu\Programs
in Vista: C:\ProgramData\Microsoft\Windows\Start Menu\Programs

On the desktop shortcuts can be found.
HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\explorer\Shell Folders
Common Desktop
in XP: C:\Documents and Settings\All Users\Desktop
in Vista: C:\Users\Public\Desktop

In the Start Menu there is a lot to be found.
HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\explorer\Shell Folders
Common Start Menu
in XP: C:\Documents and Settings\All Users\Start Menu
in Vista: C:\ProgramData\Microsoft\Windows\Start Menu
AutoCAD Startup Accelerator can be found in the Startup folder.
HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\explorer\Shell Folders
Common Startup
in XP: C:\Documents and Settings\All Users\Start Menu\Programs\Startup
in Vista: C:\ProgramData\Microsoft\Windows\Start Menu\Programs\Startup

For the current user there is this registry location that is worth looking at:
HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\explorer\Shell Folders
HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\explorer\User Shell Folders

Sometimes even if you deleted everything and then try to install again the installer says the product is already installed. Under this key: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall
you can search for the name of your AutoCAD product and then delete it. For example for AutoCAD Architecture 2013 it is HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall\{5783F2D7-B004-0000-0002-0060B0CE6BBA} and there you can also find the InstallLocation showing where the product was installed.

In some cases it can help to run a registry cleaner like CCleaner or Eusing Free Registry Cleaner.

PS. There are a lot of other places where traces are left (mostly in the registry) but usually these are the most important.
Related info: Enabling verbose logging for installation issues. Windows Installer Error Messages. How to enable verbose logging on a Windows XP-based computer (works for Vista as well).

UPDATE: How to solve problem when setup says AutoCAD is installed