I'd rightclick on the File, select Properties, makes sure it is still set to open with Excel.
If so, go to Control Panel>Programs and Features, click on the Program and see if it will Repair. You may need to insert the Disk if prompted.
If these fail, google the exact error messge verbatim to see how others have solved it.
As a last resort if other's don't have better ideas then I would uninstall in Programs and Features, after rebooting reinstall the Program.