Microsoft Outlook maintains a local disk cache of Outlook Add-ins, called Extend.DAT.

Possible Workaround if Outlook loads Wrong Add-in DLL:

-Delete the EXTEND.DAT File.

The EXTEND.DAT file includes a cache of Outlook add-ins that are selected in the Add-in Manager. Deleting EXTEND.DAT will cause Outlook to verify the selected extensions and recreate the file when Outlook is next launched.

By default, EXTEND.DAT can be found in the following locations:
-Windows XP:
\documents and Settings\<userprofile>\local settings\application data\microsoft\outlook

-Windows Vista-Windows 7:
\users\<username>\appdata\local\microsoft\outlook