After installing SQL Prompt or SQL Refactor and launching SQL Server Management Studio, the following error may appear:

Add-in failed to load: Class not registered

Possible cause

The service pack update to Windows Vista has been known to change permissions to the registry in a way that prevents user access to the information about installed classes: HKEY_CLASSES_ROOT. This prevents SQL Server Management Studio from loading the add-ins into the environment.

How to fix

To correct this problem, it's necessary to assign permissions to the registry for the logged-in user. 

1. Open the Registry Editor (regedit.exe) using the "Run As Administrator" option.
2. Right-click HKEY_CLASSES_ROOT, select "Permissions", and give the logged-in user full control.
3. The following permissions needed to be added to HKEY_CLASSES_ROOT as well:
   Users: read
   System: full control
   CREATOR OWNER: full control on subkeys only