+1  A: 

To start with something trivial: on the PC's where things didn't work, did you check the registry to see if all the entries were properly added? For instance, did the .bat file get run under an account that is actually allowed writing to HKEY_CLASSES_ROOT?

Are any of those PC's running Vista? Could it be they are running the Office suite without UAC, or with a full administrator account? In that case, the entries should be added to the HKEY_LOCAL_MACHINE.

Edit

Ok, if you are sure things are added to the registry properly, next up is RegMon (or procmon if you so prefer). Fire up regmon, start the office host application, and see where it is and isn't looking for the VSTO add-in. Start off with a filter based on your add-in name, then widen up or narrow down depending on the amount of data.

Paul-Jan
Thanks for the link. I've been twisting myself in knots to get this done. To answer your questions:1) The users are administrators on their own boxes. 2) I've moved things to HKLM from HKCU. 3) Since administrators they were able to write to HKCR. I verified those entries on several PCs.Still... no luck. I'm going insane.
Jerry
Just verified the OS version number of all of the PC's in question (if that helps):Operating system: XP Professional (5.1, build 2600) Service pack 2.Office Version: Office Professional 2003 (Outlook 2003 ver 11.8206.8221 SP3)
Jerry
I've used RegMon to see what is happening on the PCs that work vs. the PCs that don't. They both access the same set of registry entries. Both get "SUCCESSFUL" on the same lines. The only difference is that the PCs that don't work immediately set the LoadBehavior from 3 to 2. (disabled). Did I set the permissions in the CASPOL line correctly?? I assume that I did based on Microsoft forum sites.
Jerry
+1  A: 

Ok, so apparently the AddIn gets automatically disabled by Outlook. Another answer with a slightly different take:

Go to Start -> Control Panel -> System -> Advanced -> Environment Variables -> System Variables, and add VSTO_SUPPRESSDISPLAYALERTS with a value of 0. This will display error messages on screen, as opposed to silently disabling the VSTO. If you'd rather have a log to disk, add VSTO_LOGALERTS and set it to 1.

Hopefully this will provide you with some extra detail that might help us solve your issue!

Paul-Jan
Jerry
Yeah, sorry for having to go through the steps one by one as opposed to having an answer ready, but I'm glad you worked it out... well done!
Paul-Jan