Use the right-click send-to menu:
1) Open the path to your Office install (i.e. C:\Program Files (x86)\Microsoft Office\Office14)
2) Right click Excel.exe and choose "Create Shortcut".
3) Move the shortcut to %appdata%\Microsoft\Windows\SendTo (rename the shortcut if desired)
4) When you want a document to open in a new instance, right-click the file, send-to your Excel shortcut.
5) Enjoy bacon.
No regedit, gives option to open in last window, don't have to use file/open.
Source, see the solution from Swamination, second reply after the OP:
http://How to open a new instance of Excel (2010) by default when opening a file | answers.microsoft.com