In the following circumstances:
- Windows Vista with UAC activated
- VS 2008 SP1 runs as standard user even if you are an administrator (the default, you would have to use the "Run As Administrator" context menu otherwise)
- COM-based add-in (not .AddIn XML-based add-in)
- Add-in registered for all users (HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\VisualStudio\9.0\Addins)
- When the standard user tries to load such add-in not marked to load on startup through the Add-In Manager, it doesn't load and if you go back to the Add-In Manager the checkbox is still unchecked.
- When the user tries to unload such add-in marked to load on startup through the Add-In Manager, it doesn't unload and if you go back to the Add-In Manager the checkbox is still checked.
That doesn't happen for .AddIn XML-based add-ins registered for all users, only for COM-based add-ins registered for all users.