Have you got problems in AutoCAD 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 (Run regedit.exe and use with care):
For AutoCAD Architecture 2009
HKEY_CURRENT_USER\Software\Autodesk\AutoCAD\R17.2\ACAD-7004:409
For AutoCAD Architecture 2010
HKEY_CURRENT_USER\Software\Autodesk\AutoCAD\R18.0\ACAD-8004:409
For AutoCAD Architecture 2011
HKEY_CURRENT_USER\Software\Autodesk\AutoCAD\R18.1\ACAD-9004:409
For AutoCAD Architecture 2012
HKEY_CURRENT_USER\Software\Autodesk\AutoCAD\R18.2\ACAD-A004:409
Delete also the roamable contents (just paste the path into Windows Explorer):
For AutoCAD Architecture 2009
%APPDATA%\Autodesk\ACD-A 2009
For AutoCAD Architecture 2010
%APPDATA%\Autodesk\ACA 2010
For AutoCAD Architecture 2011
%APPDATA%\Autodesk\ACA 2011
For AutoCAD Architecture 2012
%APPDATA%\Autodesk\ACA 2012
Here is an explanation on where %APPDATA%\Autodesk takes you
in XP: C:\Documents and Settings\%username%\Application Data\Autodesk
in Vista: C:\Users\%username%\AppData\Roaming\Autodesk
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. For AutoCAD registry details see the documentation here. The paths for roamable contents also are different but is usually easy to figure out if you look in the %APPDATA%\Autodesk folder.