Almost a couple years ago I wrote an article The Add-In Manager of Visual Studio 2005 got broken…and won’t be fixed for SP1 and while Visual Studio 2008 fixed that problem, it has introduced a new one when running on Windows Vista, and guess what, VS 2008 SP1 doesn’t fix it either, so I guess we will have it until the next version of Visual Studio… Again, the problem happens with COM-based add-ins, not with add-ins using a .AddIn XML file. I guess that most developers of commercial add-ins still need to support Visual Studio .NET 2003 (my MZ-Tools add-in even supports Visual Studio .NET 2002) and they may want to use a single code base for the code and the setup, so they still use COM registration for add-ins for Visual Studio 2005/2008 (at least this is my case), so it would be good to get this bug fixed.

Here is the article explaining the bug:

BUG: Standard user unable to load/unload COM-based add-ins registered for all users with the VS 2008 Add-in Manager on Windows Vista
http://www.mztools.com/articles/2008/MZ2008025.aspx

And here you have the bug report on the Microsoft Connect web site to vote for it:

https://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=365846