How can I configure Excel to launch a separate window (or instance)
for each Excel document I open through explorer?

I'm aware of two different ways to do this.

#1. From the Windows "Start" menu, use the cascaded menu to open Excel.
Once Excel is running, use "File Open" to open an Excel file. Repeat the
steps for the second file.

#2. Point to an Excel file on the desktop or in a folder. With the right
mouse button, click "Send To Excel". Repeat the steps for the second
file.

For #2, you might not see "Excel" in the "Send To" menu at first. To add it
there, put a shortcut to the Excel program into the folder
C:\Windows\SendTo\

(I use Excel 97 and Windows 98.)