Excel won’t open files.
…when you double click on them but will from the File/Open menu was a weird one that I hadn’t come across before. However, following the instructions in a Google Groups posting where the tools | options | general | ignore other applications was unchecked and it worked. Very strange. Not sure why this has started to happen but it works – *shrug*

28. October 2005 at 2:51 pm :
We have had that same thing happen at work a couple of times and our IT person usually just repairs excel.