This must be such a common error, that I'm surprised it's not explicitly
"detected" by Excel on startup.

Error symptom:
Excel 2003 cannot open any vintage of .XLS file, and even on invoking Excel
alone, the same error message occurs - "This file is not in a recognizable
format.
* If you know the file is from another program, ...."
etc
[OK] [Cancel] [Help]
That's rather a cryptic error, and although it is very wordy and helpful, if
you know what's actually wrong, it could be much more informative.

Situation:
I had Office 2002 installed, and Adobe Acrobat 5.0 as well - the "full
version" which allows PDF file creation, editing, etc.
I knew that on upgrading to Office 2003, Adobe warned that the facility to
create PDF files from Word (or Excel, etc) was faulty / unsupported (or
something) with Acrobat 5.0. "Please upgrade to Acrobat 7"
So, I searched my system for all the PDFMaker DLLs, .DOT, .XLA, etc files
that were 2001-vintage (installed by Adobe Acrobat 5.0), and used WinZIP to
retain them in their current folders.
So, the file PDFMaker.xla which Office 2003 install had picked up from my
Office 2002 installed location and put into C:\Program Files\Microsoft
Office\OFFICE11\XLSTART\ was moved to a .ZIP file, but remained there.

Remedy:
Never leave anything but "normal" files that Excel understands in the
"Startup" folders.

If a PDF file is there, for instance, this error does not occur, and Excel
2003 will actually load a PDF file. But put any ZIP file into an Excel
startup folder, and the weird error message occurs and you start to wonder
about reinstalling Office 2003 / Excel 2003 all over again.

Of course, that's what I did - uninstalled then reinstalled Excel 2003. The
real remedy is to remove "foreign" files from the various XLSTART folders.

--
Ian Thomas
GeoSciSoft - Perth, Australia