Windows Vista changes the COM Activation model and will no longer create components registered in HKEY_CURRENT_USER (HKCU) if the client application is running with a Integrity Level (IL) of higher than medium. So when UAC is disabled, Outlook will run with High IL. By default, the setup project for VSTO 2005 registers the COM component in HKCU\Software\Classes. Therefore, the Addin will not be loaded by Outlook.
How does one work around this? Well that is simple you have a couple options actually.
The latter typically won't be accepted so the former will most likely be used. It's important to note that we are talking about the COM registration here, not the Addin registration. An example of an Addin registration would be HKCU\Software\Office\Outlook\Addins\<ProgID>, while COM-related registry keys are all located under HKCU\Software\Classes.
Outlook 2007 is not affected by the new COM Activation change because we use the IManagedAddin interface to load the Addin
Thanks to Misha and others for reviewing this article.