ZT: from here.
I prefer to double click on a Microsoft Excel 2007 spreadsheet and have Microsoft Excel 2007 launch a separate window for each Microsoft Excel 2007 spreadsheet that I open.
In order to force Microsoft Excel 2007 to open separate windows for each spreadsheet that you double click on, do the following:
1. Open my computer
2. Click on Tools, then Folder Options
3. Select the File Types tab
4. scroll down to the XLS extension
5. select advanced
6. Select open
7. Select Edit…
8. in the line that says “Application used to perform action:” ,
go to the end of the line and add “%1″
(that would be %1 within double quotes) so for example it may read something like
“C:\Program Files\Microsoft Office\Office12\EXCEL.EXE” /e “%1″
9. uncheck Use DDE
10. Do not make any other changes
11. Click ok and click OK again
(note if you go back into advanced and back into the open action, you will notice DDE is checked again, however the DDE message is blank; this is fine.)
12. Now scroll down to XLSX
13. repeat steps 5 through 11 above
14. Click on Close
15. Now, when you click on Microsoft Excel 2007 spreadsheets, they will open separate windows for each Microsoft Excel 2007 spreadsheet that is launched.