I got an odd error with Office 2013 Word and Excel on double clicking items to open them from Windows Explorer on Windows 8. The document opened, but there was an error window saying: “There was a problem sending the command to the program”
I tried a FixIt which appeared to sort out Word, strangely, but didn’t fix Excel.
I then followed the instructions here to turn on “Ignore other applications that use Dynamic Data Exchange (DDE)” and then turned it off again. You can find the setting in File>Options>Advanced>General.
All now seems well.