What happens when you just double click on your spreadsheet in the Document folder (without Excel being open). And also have a look once in Programs > Default Programs > Set Associations whether the file type for those spreadsheets has the right association (check the spreadsheet file type in it's Properties - they usually start with "xl...")

I had a similar problem with Photoshop; once in awhile the default "open with" program would change from PSCS2 to PS7. When I told the computer to always use PSCS2, it would ignore me (probably laughing at me), leaving the jpg extension associated with PS7.

What I would do in XP: My Computer --> View --> Options --> File Types Tab. Then, I would highlight jpg and delete it. Then I would create a new file type association for jpg (to PSCS2) and all would be well again.

But it appears that Vista has removed the ability to delete a file type extension. So the above tip would probably work if you have XP. Maybe someone here knows how to remove a file type extension association in Vista.