Since I've upgraded to Office 2007, I'm unable to double click on text files
to open them with Excel. Excel is set as the default program (although it
wasn't on the list of possibilites to select as the default). I looked at
the Excel setup and made sure that "Ignore other applications that use
Dynamic Data Exchange (DDE) was unchecked, but don't know what else could be
wrong.

I think it must be in the Excel setup somewhere, as I was always able to do
this with Office 2003.