Have you got problems with missing things like workspace, menus, ribbon, QAT, toolbars, profile, settings, etc. This tip is usually quicker than doing a repair of the installation and a solution to the problem I often used.

Notice that you will loose any of the settings and customizations that are saved either of these locations.

Delete keys like this in the Registry Editor:

For AutoCAD Architecture 2009
HKEY_CURRENT_USERSoftwareAutodeskAutoCADR17.2ACAD-7004:409

For AutoCAD Architecture 2010
HKEY_CURRENT_USERSoftwareAutodeskAutoCADR18.0ACAD-8004:409

Delete also the roamable contents (just paste the path into Windows Explorer):

For AutoCAD Architecture 2009
%APPDATA%AutodeskACD-A 2009

For AutoCAD Architecture 2010
%APPDATA%AutodeskACA 2010

Here is an explanation on where %APPDATA%Autodesk takes you
in XP: C:Documents and Settings%username%Application DataAutodesk
in Vista: C:Users%username%AppDataRoamingAutodesk

When you now start AutoCAD Architecture a so called secondary installation will happen that restores the content of the above places to what it is like if you install from scratch.

This tip can also be used for AutoCAD as well as other AutoCAD verticals.