The problem is that double-clicking a spreadsheet doesn't open a new instance
of Excel, it opens it in the current one. Then when you click the close "x"
in the upper right hand corner of the window, it closes ALL your workbooks,
not just the current one. This is not consistent with the way Word works
where opening a new Word document will open a whole new program window. Can I
change a setting to make Excel behave like Word?

I've tried this solution from to no avail:

"The answer involves changing the file association stuff in windows. Go
to explorer, choose tools, then folder options. Click the File Types
tab. Scroll down to the XLS extension. Click the Advanced button.
Choose "open", then click the edit button.

At the end of the "Application used" entry, you'll probably see: /e
After this, add: "%1" (be sure to include the quotes.)

Then uncheck the "Use DDE" checkbox. Then click OK. (Windows re-checks
it at some point for some reason, but it still works)

OK your way out of the file types dialog.

Now when you double-click a spreadsheet, it will open it in a new
instance of Excel. "