Excel Add-in will install but won't load

I have written a C# addin for Excel using the following article as a template: http://support.microsoft.com/kb/302901
It installs and loads with Excel correctly on my machine and another (non-developer) machine.
However on another user's machine it installs but doesn't load when Excel starts.

We've all got the the same version of Excel (2003 SP3).

As the Add-in is a C# one, it's a DLL not an XLA file, so it doesn't show up in Excel > Tools > Add-Ins.

Does anybody know what I need to do to get the add in working on this other machine?  Let me know if you need any more details.
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I haven't used a DLL addin before, but with a standard XLA you can have an issue if there is a fastopen switch
in the Registry addin path, under  either
HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\Excel\Add-in Manager


openshacAuthor Commented:

Thanks, but I checked and  there are not fastopen switches there in my registry.
openshacAuthor Commented:
OK, I think I may have it:

Some of the end users don't have the Office 2003 Primary Interop Assemblies installed.

openshacAuthor Commented:
OK, I needed one more thing, which I found at

2.       If your add-in is supposed to be loaded at application startup, look at the LoadBehavior for your Add-In in registry and make sure it is set to 3. This is necessary because, whenever an Add-In load fails, Office disconnects the Add-In (Loadbehavior changes to 2).

Because it failed when I didn't have the Office 2003 Primary Interop Assemblies installed I had to fix up the load behaviour.

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