Failure to fully open

I'm running Excel 2007 under XP. If I open a spreadsheet from within an Excel session, everything works as expected. If I double click on a .xls or .xlsx file, (in either Windows Explorer or Firefox), Excel opens but the spreadsheet fails to show, and I get an error message saying that the file:

could not be opened because the associated helper helper application does not exist. Change the association in your preferences.

When attempting from a Firefox attachment, the Opening Dialog box already lists MS Office Excel (default). So where would any preferences be updated?

Any ideas?

Terry