Uninstall the add-in first, then use RegEdit to find any leftover entries
and delete them. On my machine (Windows ME), they are in either of the
following 2 registry hives:

[HKEY_CURRENT_USER\Software\Microsoft\Office\9.0\Ex cel\Options]
[HKEY_CURRENT_USER\Software\Microsoft\Office\9.0\Ex cel\Add-in Manager]

In the Options part, you will see entries similar to the form:
Open = "C:\My Documents\My Add-in.xla"

I think later versions of Windows or Excel may have them inside an INI file
somewhere.
--
Regards,
Bill Renaud