A reply from Rob Bovey (via google):
This is the add-in for Novel Groupwise. It sounds like it was removed from
your computer without first removing it from the list of installed add-ins in
Excel. Choose Tools/Add-ins from the Excel menu and look for this add-in on the
list. Click it with your mouse and you should get a message saying that Excel
can't find that add-in and offering the option to permanently delete it from the
list. Choose yes and the problem should be solved.
and
http://support.microsoft.com/default.aspx?scid=kb;en-us;327856
"Gwxl97.xla could not be found" error message when you start Excel 2000