Force excel to start new instance when I double-click a workbook

[asperamanca]

Yes, I've seen this question posted all over the web.
No, I've not seen a satisfying answer so far.

I would like to:
-) Double-click any two XLS-Files in Explorer, and have them opened in
separate windows (NOT just separate taskbar entries!)

I have seen solutions that:
-) Ask me to manually open a new instance of excel and open the file.
-) Require macros
-) Change the file type settings in Explorer / Foilder options - this
didn't change anything!
-) Check the option "Ignore other applications" - suddenly my regular
files will not open anymore
-) Instruct me how to get separate taskbar entries - that's not what I
need
-) Require me to manipulate single file shortcuts - I want a solution
that works by default

I dimly remember a solution exists - something about a registry
setting.

Anyone in the know?

Thanks!

Robert