I put this in a reply to a related post which may confuse people. It
should be in it's own topic.

I think I have figured out this stupid Excel thing.

Excel has this bast%^%^ized MDI SDI thing.

I think most people run Excel with it's child Window Maximized (which
makes it really confusing).

So when you click on another file it switches to that file, hides the
other window but adds another icon on your task bar giving you the
feeling you started another instance of excel (but it didn't !!). I've
been burned by this too many times and shutdown what I thought was one
instance of excel turned out to be all of them. Word does not behave
this way.

So there is an option to disable this feature called "Ignore Other
Applications" (under Options- General). If you look at the help on
this it basically shuts of DDE. What DDE basically does is when you
click on a file it checks to see if Excel is already running and if it
is, it passes the file to that already running instance. The problem
with turning this switch on is the file mappings that are already
installed assume DDE is on and things break. So when you shut that
option off you suddenly start getting errors that Excel cannot find the

file.

To fix this go into your File Explorer (My Computer) and Choose Folder
- Options - File Type. Find XLS and select it, then click advanced
button. Now Select the "Open" Action and choose Edit button. Now this
is the trick, shutoff DDE by unchecking it. Then edit the end of the
action. The end should be something like this (note the "%1") and note
there must be a space between the /e and the first quote.

"C:\Program Files\Microsoft Office\OFFICE11\EXCEL.EXE" /e "%1"

Now this allows you to create a SEPERATE instance of excel everytime
you click on an Excel file. And when you close that excel instance it
has no impact on any other. It also gets rid of the I can't find the
file problem. In some cases you will see a %1 without the quotes. That
causes the problem that excel can't find files when the path has spaces
in it.

You may want to repeat this for other files types that maps to Excel.
Like .CSV

If you have "ignore other applications" switch unchecked (default
behavior) you get the funky SDI/MDI/Excel Instance behavior.

If you have "ignore other applications" switch checked, you get the
more logical MDI/SDI/Excel Instance behavior but the file type mappings
must be fixed as I described above.

This all assumes under WinXP.