xl2k was the first version to offer Tools|Options|view tab|windows in taskbar.

It showed each window as a different icon on the taskbar--but all those windows
were in the same instance of excel.

If you wanted separate instances of excel for each workbook, this may work...

You can change this setting (xl2003 and below menu):
Tools|Options|General tab|Ignore other applications (check it)

Be aware that one side effect of having this setting checked is sometimes you
can't open a file by double clicking on it in explorer.  (toggle it back and
that should be fixed.)

This shows up for workbooks that have spaces in their name or path:
c:\my documents\excel\testworkbook.xls
or
c:\test of workbook2.xls

Cannot find C:\my, then more "Cannot find ..." messages.

It's more trouble than it's worth (for me, anyway).

If I want a second instance of excel, I'll just use:
windows start button|run
excel
then file|Open to open my file.

But I rarely want separate instances of excel.  It makes it more difficult to
copy|paste between workbooks.