I have the same problem. If I save a file to the desktop (eg a pdf file) the next time I open Excel it will open the pdf file (badly I might add, just garbled text and images). It will do the same with text and image files, or attempt to. Trashing the files has no effect. Repairing permissions has no effect. (In fact I find repairing permissions in general to be a fairly useless pastime when it comes to sorting out problems, and yet everyone seems to recommend it).
Even trashing the files it attempts to open, and emptying the trash, has no effect - Excel still seems to dig them up from somewhere.
So I sympathise with your problem CrazyHorse, but as yet haven't found an answer.