If you get this message "There was a problem sending the command to the program" when double clicking on excel files, but can launch excel itself, then you should try this solution.
1. Start Excel
2. Click on the Excel logo on the top left
3. Choose Excel Options
4. Select Advanced tab on the left menu
5. Scroll down to the "General" section
6. Uncheck Ignore other application that use Dynamic Data Exchange (DDE) option
7. Click OK, and try opening excel files again.