So you bought a new computer and it came with Outlook 2013 installed as a trial and you used it for a bit and hated it and decided to revert to Outlook 2010 and all of a sudden your COM addins went mad and started throwing horrible looking errors that said things like:
Unable to cast COM object of type ‘Microsoft.Office.Interop.Outlook.ApplicationClass’ to interface type ‘Microsoft.Office.Interop.Outlook._Application’. This operation failed because the QueryInterface call on the COM component for the interface with IID ‘{00063001-0000-0000-C000-000000000046}’ failed due to the following error: Library not registered. (Exception from HRESULT: 0x8002801D (TYPE_E_LIBNOTREGISTERED)).
You or your IT people tried everything but to no avail….
The problem is caused by some Registry entries which need to be edited. Here’s how to fix it.
1) Open Regedit and find HKEY_CLASSES_ROOTInterface{00063001-0000-0000-C000-000000000046}TypeLib
2) Find a ‘(Default)’ entry with the value ‘{00062FFF-0000-0000-C000-000000000046}’ and make sure the “Version” Data is set to 9.3 for Outlook 2007, 9.4 for Outlook 2010 or 9.5 for Outlook 2013.
3) Now navigate to HKEY_CLASSES_ROOTTypeLib{00062FFF-0000-0000-C000-000000000046} and delete the version entry 9.3, 9.4 or 9.5 corresponding to the versions of Outlook you are NOT using.
Problem solved!
It’s always a good idea to reboot after making changes to the Registry.