Windows – VSTO Outlook AddIn disabled in Vista

Solved my problem after weeks of pain. The "Manifest" reg key was getting corrupted to some junk value during the setup build. It was a known Visual Studio bug that supposedly got fixed in Visual Studio 2008 SP 1, but apparently wasn't for me. Renaming the project name to be different from the plugin name fixed the problem. Random, huh?

Here's a bit more detail on RobertG5's solution (too long for a comment):

The problem was that the add-in had been hard disabled by Outlook. As I've learned, that's something different than the "usual" not-loading scenario. The key to realizing this was to notice that the add-in did not show up under Inactive Application Add-Ins, but rather under Disabled Application Add-Ins. That makes a difference: In the latter case, just going to the COM-AddIn screen and ticking the check box just does nothing. (I guess a nice message box "You cannot load this add-in because it has been hard-disabled" would be too much to ask... sigh.)

So, how do I re-enable a hard disabled add-in?

1. In the Manage box, change COM Add-ins to Disabled Add-ins, and then click Go.
2. Select the add-in and click Enable. Click Close.

OK, now the add-in can be loaded again:

1. In the Manage box, change Disabled Add-ins to COM Add-ins, and then click Go.
2. select the check box next to the disabled add-in. Click OK.

Reference: http://msdn.microsoft.com/en-us/library/ms268871.aspx