How do I force Excel (and other Microsoft Office products) to stop opening files in the same application?

I have Office 2010 and the only app that does this for me is Excel.

A posting on PC Review, How to open vista excel in multiple windows versus 1 window? explains how to set this up:

1. Use the File Association Settings
2. You have to tell Excel not to use DDE to open the worksheet.
3. Open Windows Explorer, Tools, Options, File Types
4. Scroll down to XLS
5. Select Advance Button
6. Click on OPEN in the Actions window, Then Edit
7. On the 2nd line for application used add "%1".
8. Make sure to put the quotes around %1.
9. Example ...\Excel.exe" /e "%1"
10. Write down what you see in the DDE Section.
11. You will need this information if you want to return to the original settings.
12. Now DeSelect Use DDE

When you double click on any XLS file a separate instance of excel will run.

Another advantage is that you can open more than one XLS file with the same name

I have been using a small batch file to circumvent this problem with Excel and it works fine for me: every time I double click a document, it opens another instance of Excel.

1. Using notepad create a batch file (e.g., "C:\Program Files\ExcelLauncher.bat") with following content:

   START "" "C:\Program Files\Microsoft Office\Office14\EXCEL.EXE" %1
   
   START "" "C:\Program Files\Microsoft Office\Office14\EXCEL.EXE" /x %1         (for Excel 2013)
   

2. Instead of "Office14" write the folder name where your Excel is installed (it depends on the version of the Office). It can be "Office12" ili "Office11" for older versions.

3. Try double clicking the batch file. If it is configured works correctly, it should open Excel.

4. Set up Excel documents to be opened with this batch file (right click any Excel document -> Open with -> Choose default program -> find and select "ExcelLauncher.bat").

Warning - this might change the desktop icon for Excel documents. Use "Default Programs Editor(free)" and "BeCyHome(free)" to re-apply the correct Excel icon.

http://www.howtogeek.com/109347/extract-high-quality-icons-from-files-using-a-free-tool/ http://www.winhelponline.com/blog/default-programs-editor-change-file-type-icon-windows-7-vista/

Note that you need to make 2 separate batch files to have separate icons for .xls and .xlsx. Otherwise, only a single icon can be applied to both filetypes.

In my experience, if you open the second spreadsheet using File >> Open, it opens in the same program window.

If you open a second Excel instance from the Start menu or a desktop icon or such, then use File >> Open in that window, it opens in its own program window.

I agree that it's broken behavior and I know of no way to fix it except, perhaps, begging Microsoft to change it in some future release.