This happened to me and I finally found the fix today!!

the problem was that in the options of excel, in the general tab, the
"ignore other applications" was ticked on. problem solved by ticking it off.



"mswlogo" wrote:

Correction:

The problem with turning THIS SWITCH ON is the file mappings that are
already
installed assume DDE is on and things break.

If you have it unchecked (default behavior) you get the funky
SDI/MDIExcel Instance behavior.

If you check it, you get the more logical MDI/SDI/Excel Instance
behavior but the file type mappings must be fixed as I described above.